Actually JkHenry, If you're trying to find AC, you have to plug in "x." First, set 3x+3 and 5x-15 equal. 5x-15=3x+3. Then, add 15 to both sides and you'll have 5x=3x+18. Now, subtract 3x from both sides, getting this result; 2x=18. Now there's not much to go. Simplify 2x=18 to x=9. Now plug in 9 as "x" to the equations and you should get the same answer for both of them. 5*9=45. 45-15= 30. 3*9=27. 27+3=30. :D