13.2 The coordinates of the points $P$ and $Q$ are $(a, 2)$ and $(x,-6)$ respectively.
If the coordinates of the mid-point of $P Q$ is $(18, b)$, find $a$.
13.3 A man travels from $X$ to $Y$ at a uniform speed of $a \mathrm{~km} / \mathrm{h}$ and returns at a uniform speed of $2 a \mathrm{~km} / \mathrm{h}$. If his average speed is $c \mathrm{~km} / \mathrm{h}$, find $c$.

I3.4 If $f(y)=2 y^{2}+c y-1$, find $f(4)$.
$$
\begin{array}{l}
\frac{1}{2}(a+30)=18 \\
a=6
\end{array}
$$

Let the distance between $X$ and $Y$ be $s \mathrm{~km}$.
$$
\begin{array}{l}
c=\frac{2 s}{\frac{s}{a}+\frac{s}{2 a}}=\frac{2}{\frac{1}{6}+\frac{1}{12}}=8 \\
f(4)=2(4)^{2}+8(4)-1=63
\end{array}
$$