Let f(x) = x^3 - 12x - 7.2

Note that f(x) is continuous for all values of x. Apply the Intermediate Value Theorem:

f(0) < 0 and f(4) > 0 => f(a) = 0 for 0 < a < 4.

f(-4) < 0 and f(-3) > 0 => f(b) = 0 for -4 < b < -3.

f(-3) > 0 and f(-1/2) < 0 => f(c) = 0 for -3 < c < -1/2.

Therefore two negative roots x = b and x = c and one positive root x = a.