Solve the quadratic by quadratic formula below

a^2-4a=21

a^2 - 4a = 21

by trasponsing 21 to the left:

a^2 - 4a - 21 = 0

by quadratic formula:

x = [ - (-4) ± sqrt [ ( - 4) ^2 - 4 (1) (-21) ]] / 2 (1)

x = [4 ± sqrt (100) ] / 2

x = [4 ± 10] / 2

x = - 3, 7