Can someone pls help me with this Pls

How do you use the additive inverse to evaluate an expression that uses subtraction in a full sentence

On a architect's drawing of a house, 1 inche representa 1.5

Kapil needed to buy a long wooden beam. he went to two sawmills that each charge an initial fee plus an additional fee for each meter of wood. the following equation gives the price (in dollars) of a wooden beam from the first sawmill as a function of its length (in meters). p = 5+20xp=5+20x