1.

$OABC$ is a parallelogram.

$\overset{\rightarrow}{OA}=aand\overset{\rightarrow}{OC}=c$.

$X$ is the midpoint of the line $OB$.

$OAD$ is a straight line such that $OA:AD=k:2$.

Given that $\overset{\rightarrow}{XD}=2a-\frac{1}{2}c$, find the value of $k.$