$$5\times 7+5\times \left(-8i\right)+6i\times 7+6\left(-8\right)i^{2}$$
By definition, $i^{2}$ is $-1$.$$5\times 7+5\times \left(-8i\right)+6i\times 7+6\left(-8\right)\left(-1\right)$$
Do the multiplications.$$35-40i+42i+48$$
Combine the real and imaginary parts.$$35+48+\left(-40+42\right)i$$
Do the additions.$$83+2i$$