To simplify this, use FOIL (First outer, Inner last) which would be distributing each term to one another.
This would look like this:
(-7*-6)+(-7*-6i)+(-4i*-6)+(-4i*-6i)
When you simplify it should look like this:
(42)+(42i)+(-24i)+(24(i^2))
Now for the last term, since i is equivalent to √-1, i^2 would be -1
This would look like this: (24*-1)=-24
Now that we have everything simplified, combine like terms and your answer is 18+18i