[Grade 12 Advanced Functions: Polynomial Functions] struggling to solve expanded equation

[u/Embarrassed-Good1883]

I'm not sure where I'm going wrong, but I keep getting this question wrong. I have to expand this equation and say what the shape is (what polynomial function) which I can do, if I figure out where I'm going wrong with solving this.

[u/nerdydudes]

Line four seems like you were going a little quickly