-8i(2i-7)

All these complex value problems can be treated just like polynomials, where the reals and imaginaries have to be treated separately

-8i(2i) - 8i(-7)
-16i^2 + 56i
16 + 56i

Don't forget that i^2 = -1
also key: (a+bi)(a-bi) = a^2 - b^2i^2 = a^2+b^2