Normally if you want to get the f(3) you would replace every x value by in. For this question you have to change each x value to the g(x) value. g(x) = 12-5x, which means you can replace the g(x) by 12-5x in the f(g(x)).
f(g(x)) = f(12-5x)
f(12-5x) = 1/8(12-5x)+3
f(12-5x) = 1.5 - 0.628x + 3
f(12-5x) = 0.628x + 3.5
f(g(x)) = 0.628x + 3.5
