The reasoning behind why it equals 19 is because you use PEMDAS (aka: Order Of Operations). When you solve, you solve in the same order PEMDAS, so you first solve the parenthesis. Next, you'd solve exponents, but in this case, there are none. So, you solve the multiplication and/or division (from left to right). Finally you'd solve completely, by adding and subtracting (from left to right). Doing all these steps would give you your answer of 19.
I hope this helps!
