Given DE || BC, find the length of BD

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

Nine more than four times a number is the same as one less than twice the number. find the number?

I’m really confused and need to learn this