Hello!
[tex]\large\boxed{a = 0, -3, 7}[/tex]
a³ = 4a² + 21a
Move all "a" to one side:
0 = -a³ + 4a² + 21a
Factor out "-a" from the equation:
0 = -a(a² - 4a - 21)
Solve for a by factoring:
0 = -a(a - 7)(a + 3)
Solve for the roots by setting each factor equal to 0:
-a = 0
a = 0
a - 7 = 0
a = 7
a + 3 = 0
a = -3
Solutions: a = 0, -3, 7
