3(4a+9)-10(a-6) (distribute) 12a+27-10a+60 (add common terms) 2a+87 (have x be alone on one side of the equation, in this case by subtracting 87 from both sides) 2a=-87 (divide both sides by 2) a=43.5
F(g(x)) means you take the g function and put it into x in the f function. He took the function and put it into the g, which was the incorrect order. He should have had this: