what is the solution set for 3y-4x=40 x-2y= -15

From the second of these we have x = 2y - 15  (by adding 2y to both sides).

 

Replace x in the first equation by 2y - 15

3y -4(2y - 15) = 40

3y - 8y +60 = 40

-5y + 60 = 40

Add 5y to both sides

60 = 40 + 5y

Subtract 40 from both sides

20 = 5y

Divide both sides by 5

y = 4

 

Using the fact that x = 2y - 15 we have 

x = 2*4 - 15

x = -7