Substitution involves replacing part of one equation with the other equation. In this case, the second equation is already solved for y. All we have to do is replace y in the first equation with 12x+3.
8x - (1/3)(12x+3) = 0
Distribute the -1/3
8x - 4x - 1 = 0
4x - 1 = 0
4x = 1
x = 1/4
Now plug 1/4 in for x to solve for y.
12(1/4) + 3 = y
y = 6
Answer: (1/4, 6)