Place (3x + 1)/12, and (4x)/15 (Note: the bottom triangle's base is 12 + 3)
Set the two equal to each other
(3x + 1)/12 = (4x)/15
Cross multiply
15(3x +1)/12(12) = 12(4x)/15(15)
15(3x + 1) = 12(4x)
Distribute the 15 to the two terms in the parenthesis, and 12 to 4x
15(3x) = 45x
15(1) = 15
12(4x) = 48x
45x + 15 = 48x
Isolate the variable, subtract 45x from both sides
45x (-45x) + 15 = 48x (-45x)
15 = 48x - 45x
Simplify
15 = 3x
Isolate the x, divide 3 from both sides
15/3 = 3x/3
x = 15/3
x = 5
5 is your answer
hope this helps