1.

$PQRS$ is a parallelogram with diagonals $PR$ and $PR$ intersecting at $X$.
$\overset{\rightarrow}{PQ}=a$ and $\overset{\rightarrow}{PS}=b$.

Find $\overset{\rightarrow}{QX}$ in terms of $a$ and $b$.
Give your answer in its simplest form.

$\overset{\rightarrow}{QX}=$ ...............................................