Starting system: 

Multiply the top equation by
5 and the bottom equation by 3: 

Add the terms in each column: 
0x + 11y = 60 + 39 11y = 99 y = 9 
Substitute "9" for "y" in the top original equation, to get:
3x  4(9) = 12
3x = 36  12 = 24
x = 8