okay so first
you have the constant (4) to the other side of the equation and change the sign.
since you have x=7+mn-4 now you gave to collect like terms (7-4).
leaving the equation x=3+mn
to finish the equation you have to rewrite the equation to parametric form. x=3+mn, m to R (all real numbers), n to R (all real numbers) hope it helped
