The diagram shows a triangle $OAB$ and a straight line $OAC$.

$OA:OC=2:5$ and $M$ is the midpoint of $AB$.
$\overset{\rightarrow}{OA}=a$ and $\overset{\rightarrow}{OB}=b$.

Find $\overset{\rightarrow}{AB}$, in terms of $a$ and $b$, in its simplest form.

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

1.2. Find $\overset{\rightarrow}{MC}$, in terms of $a$ and $b$, in its simplest form.

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