This is a simultaneous system of two linear equations with two unknowns, as the number of unknowns equals the number of equations, we can solve it:
y = 5x + 21
y = -7x - 15
we can equate both equations, given the fact that both are equal to y:
y = y
5x + 21 = -7x - 15
and solving for x:
5x + 7x = -15 - 21
12x = -36
x = -36/12
x = -3
then we substitute this result in the original equation:
y = 5x + 21
y = 5(-3) + 21
y = -15 + 21
y = 6
hence the solution is x = -3 and y = 6
