Answer
$7$.
Work Step by Step
The given expression is
$(x+3y)(x-?y)=x^2-4xy-21y^2$
Factor the right hand side:
$=x^2-4xy-21y^2$
$=x^2-7xy+3xy-21y^2$
$=x(x-7y)+3y(x-7y)$
$=(x+3y)(x-7y)$
Plug into the given expression.
$(x+3y)(x-?y)=(x+3y)(x-7y)$
By comparing both sides, the question mark is $7$.
