gonna start with the second equation first so y = (5-10x)/15
subbing that into the first equation 15x - (4/3)(5-10x)=-7
15x - 20/3 + (40/3)x = -7 making 15x into 45/3 x and -7 into -21/3 then adding them together
(85/3)x = 1/3 multiply everything by 3
85x = 1 so x = 1/85 and plug that into either equation to solve for y
15(1/85)-20y = -7 made 15/85 into 3/17
20y = -7 - 3/17 multiplying by 17 to the whole problem
340y = -119-3
340y = -122
y = -61/170
enjoy
+18 
