If the simple interest earned on $6000 for 9 years is $2,160. Then what is the interest rate? Please explain so I know how to solve problems like these in the future.

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.