maths question thanks :)

topic of simultaneous equations: one quadratic and one linear

so first step is to rearrange the linear equation but how do i know if i make y the subject or if i make x the subject?

i did the whole thing by rearranging the linear equation to make y the subject but i got stuck halfway through and i checked the mark scheme and they’d made x the subject of the linear simultaneous equation. how do i know which one to do??