Could someone please explain how to resolve the forces? I dont understand how to go about solving this question. Cheers.
You have the magnitudes and angles with the x-axis of each vector. To find the resultant vector you need to convert these into vector components, and add them all together.
So first of all solve for the x-component:
$\displaystyle 1275+4000\cos(45)+5000cos(150)+0=-226.7N$
Next for the y-components:
$\displaystyle -3000+4000\sin(45)+5000\sin(150)+0=2328.4N$
So your resultant force vector is:
$\displaystyle \left(\begin {array} {cl} -226.7 \\ 2328.4 \end {array} \right)$
And you can calculate the magnitude using pythagoras, and the direction using basic trig.
Once you have these:
$\displaystyle F=ma\implies a=\frac{F}{m}$
Will give you the magnitude of your acceleration. And its direction will be the smae as that of the force vector.