It isn't easy really but you can always divide the number out. For example 50% of 200 is 100 by dividing 200 by 2 or 25% of 100 is 25 by dividing 100 by 4 since 25% is equivalent to 1/4
F (-3) = -3(-3) -4
F(-3) = 9 - 4
F(-3) = 5
(5,-3) is the ordered pair