Your answer is correct. But it is not a unique answer. It is not a unique answer, because there is more than one pair of such straight lines. For example, the pair (the system of equation) x= 3 y= -4 is another pair (another system of equations). Any system of equations (and any pair of perpendicular lines) that obtained from your pair (or from my pair) by a rotation to an arbitrary angle about the center (3,-4) satisfies the given conditions/requirements, too.