Is it true that the order when dividing three real numbers does not effect the answer?

F(x)=x/2-2 and g(x)=2x^2+x-3 find (f+g)(x)

Astuntman jumping off a 20-m-high building is modeled by the equation h=20-5t^2, where t is the same in seconds. a high-speed camera is ready to film him between 15m and 10m above the ground. for which interval of time should the camera film him?

Another type of subtraction equation is 16-b=7. explain how you would sole this equation then solve it.