Well I got my Math-Finals tomorrow and have just been doing some exercises. But I'm stuck on this part. Maybe someone does understand it on here

When I got this calculation:


I don't get the part inside the [] brackets
I know that they made it something like
x-3/4 = sin(φ)
but how did they mix it with the cos²(φ) to -((x-3)/4)²+1 ?


Would be cool if someone could help me