You can treat this as a proportion. (3x+3)/12 = (4x+5)/18. Cross multiply results in 54x+54 = 48x+60. Subtract 54 from both sides 54x = 48x+6. Subtract 48x from both sides 6x = 6. x=1. When we substitute 1 for x, we see the proportion (3(1)+3)/12 = (4(1)+5)/18 results in 6/12 = 9/18. Both simplify to 1/2
