Help please. I need answers

Suppose that the price p, in dollars, and the number of sales, x, of a certain item follow the equation 4 p plus 4 x plus 2 pxequals56. suppose also that p and x are both functions of time, measured in days. find the rate at which x is changing when xequals2, pequals6, and startfraction dp over dt endfraction equals1.5.

There are 93 calories in a small candy bar how many calories are ther in a half dozen small candy bars?

How are algebraic expressions evaluated using substitution?