Y=5x+21 and y=-7x-15

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