320
1) To evaluate an expression is to see what is the corresponding value of it when we plug into that another value.
2) So, in this case, let's plug into that expression a= -7 and b= 4
[tex]\begin{gathered} 2a^2-8ab-\sqrt[]{b} \\ 2(-7)^2-8(-7)(4)-\sqrt[]{4} \\ 2\cdot49+224-2 \\ 320 \end{gathered}[/tex]
3) Hence, the answer is 320
