Given acute triangle $\triangle ABC$ in plane $P$, a point $Q$ in space is defined such that $\angle AQB = \angle BQC = \angle CQA = 90^\circ.$  Point $X$ is the point in plane $P$ such that $QX$ is perpendicular to plane $P$. Given $\angle ABC = 40^\circ$ and $\angle ACB = 75^\circ,$ find $\angle AXC.$
1. **Understanding the Problem:**
   We are given an acute triangle $\triangle ABC$ in plane $P$ and a point $Q$ in space such that $\angle AQB = \angle BQC = \angle CQA = 90^\circ$. This implies that $Q$ is the orthocenter of the triangle formed by the points $A, B, C$ in space. Point $X$ is the projection of $Q$ onto plane $P$, meaning $QX$ is perpendicular to plane $P$.

2. **Identifying Key Points:**
   Since $Q$ is the orthocenter of the triangle formed by $A, B, C$ in space, and $X$ is the projection of $Q$ onto plane $P$, $X$ must be the orthocenter of $\triangle ABC$ in plane $P$. This is because the orthocenter of a triangle is the point where the altitudes intersect, and the projection of the orthocenter in space onto the plane of the triangle is the orthocenter of the triangle in that plane.

3. **Using Known Angles:**
   We are given $\angle ABC = 40^\circ$ and $\angle ACB = 75^\circ$. We need to find $\angle AXC$.

4. **Using the Orthocenter Property:**
   A well-known property of the orthocenter in a triangle is that $\angle AXC = 180^\circ - \angle ABC$. This property arises because the orthocenter, the circumcenter, and the vertices of the triangle form cyclic quadrilaterals, leading to supplementary angles.

5. **Calculating $\angle AXC$:**
   \[
   \angle AXC = 180^\circ - \angle ABC
   \]
   Substituting the given value of $\angle ABC$:
   \[
   \angle AXC = 180^\circ - 40^\circ = 140^\circ
   \]

The final answer is $\boxed{140^\circ}$.