ANSWER:
a) 4t
b) 0
STEP-BY-STEP EXPLANATION:
We have the following expression:
[tex]f\mleft(x\mright)=4x-12[/tex]a) We evaluate when x = t + 3, just like this:
[tex]\begin{gathered} f(t+3)=\:4\cdot\left(t+3\right)\:-\:12 \\ \\ f(t+3)=4t+12-12 \\ \\ f(t+3)=4t \end{gathered}[/tex]b) We evaluate when x = 3, just like this:
[tex]\begin{gathered} f(3)=4\cdot3-12 \\ \\ f(3)=12-12 \\ \\ f(3)=0 \end{gathered}[/tex]