Substitution

T(n) = O(n³) $\leq$ cn³, Assume T(n/2) $\leq$ c(n/2)³

T(n) $\leq$ 2c(n/2)³ + n³

= 1/4cn³ + n³

= (1 + c/4)n³

$\leq$ cn³ when

1 + c/4 $\leq$ c, 3/4c $\geq$ 1, c $\geq$ 4/3

T(n) = $\Omega$(n³) $\geq$ cn³, T(n/2) $\geq$ c(n/2)³

T(n) $\geq$ 2c(n/2)³ + n³

= 1/4cn³ + n³

= (1 + c/4)n³

$\geq$ cn³ when

1 + c/4 $\geq$ c, 3/4c $\leq$ 1, c $\leq$ 4/3

Thus, T(n) = $\Theta(n^3)$.