So, it is a system of two linear equations with two unknowns, therefore we can solve it:
4x - 7y = 5
9x – 7y = –15
we multiply the first equation by -1 and then add both equations together:
-4x + 7y = -5
9x - 7y = -15
-----------------
5x + 0 = -20
x = -4
and now we substitute this result in the first equation:
4x - 7y = 5
4(-4) - 7y = 5
-16 - 7y = 5
7y = -16 - 5
7y = -21
y = -3
