this is a binomial. You can multiply it out by substituting 7i-4j for a, and 4i+3j for b.
(7i-4j)(4i+3j)
you come up with: 28i^2 + 21ij - 16ij -12ij
this then comes out to be 28i^2 -7ij
this is an odd choice of variables for a problem.. let me know if this is the answer you were looking for.