Try using a substitution $y = x + a$ for a suitable value of $a$ to get it in a form that's amenable to the result from the previous part. Work withthe general $a$ and then choose a value for it once it becomes obvious what's the best value. This sort of 'substitution' is a recurring technique in STEP.

No worries, it was a very tiny hint, so well done for getting it out on your own.

This sort of idea: a linear substitution, whilst seemingly blindingly obvious now is actually a fairly important theme. It's actually the cornerstone of the solution of the general cubic. del Ferro, a mathematician from long ago knew the general solution to the equation $ax^3 + bx + c = 0$ which isn't quite the general cubic. But it turns out that for any cubic equation, it's possible to use a linear substitution $x = a'y + b'$ to get rid of the $x^2$ term. This means that the solution for the general cubic turns to the solution of something we already knew how to solve.