Answer:
= 8x^2 + 12xy − 56y^2
Step by step:
(2x+7y)(4x−8y)
=(2x+7y)(4x+−8y)
=(2x)(4x)+(2x)(−8y)+(7y)(4x)+(7y)(−8y)
=8x^2−16xy+28xy−56y^2
=8x^2+12xy−56y^2
