7x + 5y = 29
21x - 25y = -33

    5*(7x + 5y = 29) 
+  21x - 25y = -33

    35x + 25y = 145
+  21x - 25y = -33
   56 x + 0y = 112

56x = 112
x = 2

Plug the x back in to the original equation to solve for y. 

7x + 5y = 29
7(2) + 5y = 29
14 + 5y = 29
5y = 15
y = 3

21x - 25y = -33
21(2) - 25y = -33
42 - 25y = -33
-25y = -75 
y = 3

x = 2 and y = 3