You use to say "y=2x+3;solve for y when x = -1." now you say "f(x) =2x +3;find f(-1)."pronounced as"f-of-x is 2x plus three; find "f-of-x-negative-one.
Yoy do examly the same with the other case. You plug in-1 for x multiply by 2 and then add the 3, simplifying to get the value +1 diba!! Ang dali