If the function f (x) = x ^ 3 + 2x ^ 2-ax + 3 has both minimum and maximum, then the value range of a is?

If the function f (x) = x ^ 3 + 2x ^ 2-ax + 3 has both minimum and maximum, then the value range of a is?

f(x)=x^3+2x^2-ax+3
f'(x) = 3x^2 + 4x - a
f'(x) = 0
3x^2 + 4x - a =0
△ >0
16+ 12a > 0
a > -4/3