$$18-3\left(-7+6i\right)$$
Multiply $3$ times $-7+6i$.$$18-\left(3\left(-7\right)+3\times \left(6i\right)\right)$$
Do the multiplications in $3\left(-7\right)+3\times \left(6i\right)$.$$18-\left(-21+18i\right)$$
Subtract $-21+18i$ from $18$ by subtracting corresponding real and imaginary parts.$$18-\left(-21\right)+18i$$
Subtract $-21$ from $18$.$$39-18i$$