The expression is telling you to proceed as follows:
- Pick some number [tex] y [/tex]
- Pick some number [tex] a [/tex]
- Compute the square of [tex] y [/tex]
- Multiply it by 4
- Subtract [tex] a [/tex] from this result.
So, first of all, there's no point in fixing [tex] x = 2 [/tex], because x doesn't appear in the expression. As for the evaluation, let's follow the aforementioned steps with fixed values:
Pick some number [tex] y [/tex]
- Pick [tex] y=-3 [/tex]
- Pick [tex] a=6 [/tex]
- Compute [tex] y^2 = (-3)^2 = 9 [/tex]
- Multiply it by 4: [tex] 9 \cdot 4 = 36 [/tex]
- Subtract [tex] 6 [/tex] from this result: [tex] 36-6=30 [/tex]
