diketahui rumus f(x)=4x-9 jika f(a) = - 21, maka f(a +21)adalah....

f(x) = 4x - 9
f(a) = 4a - 9
-21 = 4a - 9
-4a = 12
a = -3

f(a+21) = 4a - 9
f(a+21) = 4 (a+21) - 9
••••••• = 4 (-3+21) - 9
••••••• = -12 + 84 - 9
••••••• = 63