What is the difference of outputs for any two inputs that are one value apart

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

Determine which expressions represent real numbers and which expressions represent complex number. asaaap! plis!

Ineed to know the answer to this algebra question.