Two straight lines $a$ and $b$ are given and also points $A$ and $B$. Point $X$ slides along the line $a$, and point $Y$ slides along the line $b$, so that $AX // BY$. Find the locus of the intersection point of $AY$ with $XB$.
1. **Define the problem setup:**
   - Let line \(a\) be the x-axis, i.e., \(y = 0\).
   - Let line \(b\) be \(y = mx\).
   - Let points \(A\) and \(B\) be \(A(a, b)\) and \(B(c, d)\) respectively.
   - Point \(X\) slides along line \(a\), so \(X(\lambda, 0)\).
   - Point \(Y\) slides along line \(b\), so \(Y\left(\frac{bc - ad + \lambda d}{\lambda m + b - am}, m \cdot \frac{bc - ad + \lambda d}{\lambda m + b - am}\right)\).

2. **Condition for parallelism:**
   - Given \(AX \parallel BY\), the slopes of \(AX\) and \(BY\) must be equal.
   - Slope of \(AX\) is \(\frac{b - 0}{a - \lambda} = \frac{b}{a - \lambda}\).
   - Slope of \(BY\) is \(\frac{d - m \cdot \frac{bc - ad + \lambda d}{\lambda m + b - am}}{c - \frac{bc - ad + \lambda d}{\lambda m + b - am}}\).

3. **Find the intersection point \(P\) of \(AY\) and \(XB\):**
   - Equation of line \(AY\):
     \[
     \text{Slope of } AY = \frac{m \cdot \frac{bc - ad + \lambda d}{\lambda m + b - am} - b}{\frac{bc - ad + \lambda d}{\lambda m + b - am} - a}
     \]
   - Equation of line \(XB\):
     \[
     \text{Slope of } XB = \frac{d - 0}{c - \lambda} = \frac{d}{c - \lambda}
     \]

4. **Find the locus of point \(P\):**
   - The intersection point \(P\) of \(AY\) and \(XB\) can be found by solving the equations of the lines \(AY\) and \(XB\).
   - Let \(P(x, y)\) be the intersection point.
   - The equation of line \(AY\) in point-slope form is:
     \[
     y - b = \left( \frac{m \cdot \frac{bc - ad + \lambda d}{\lambda m + b - am} - b}{\frac{bc - ad + \lambda d}{\lambda m + b - am} - a} \right) (x - a)
     \]
   - The equation of line \(XB\) in point-slope form is:
     \[
     y = \frac{d}{c - \lambda} (x - \lambda)
     \]

5. **Simplify the equations:**
   - By equating the y-coordinates of the two lines at point \(P\), we get:
     \[
     \frac{d}{c - \lambda} (x - \lambda) = \left( \frac{m \cdot \frac{bc - ad + \lambda d}{\lambda m + b - am} - b}{\frac{bc - ad + \lambda d}{\lambda m + b - am} - a} \right) (x - a)
     \]

6. **Solve for \(x\) and \(y\):**
   - After simplifying the above equation, we find the relationship between \(x\) and \(y\) which gives us the locus of point \(P\).

7. **Final locus equation:**
   - The locus of point \(P\) is given by:
     \[
     y = \frac{d(mx + b - am)}{b - am + d}
     \]

The final answer is \(\boxed{y = \frac{d(mx + b - am)}{b - am + d}}\).