In parallelograms, opposite sides are congruent, meaning that they are the same lengths. So, we have:
y - 4x = 9
-x + 4 = y
Add x to both sides of the second equation:
y + x = 4
y - 4x = 9
This is a system of linear equations. Solve by eliminating the y and subtracting the two equations:
5x = -5
x = -1
Use x to solve for y:
y + (-1) = 4
y = 5
So, x = -1 and y = 5.