Answer :

whats the equation? but when ever you see f( ), what ever is in the parenthesis just plug into x
 f(x) = x² - 4x + 3
f(-1) = (-1)² - 4(-1) + 3
f(-1) = 1 + 4 + 3
f(-1) = 5 + 3
f(-1) = 8