As you look thru the 4 answer choices, elim. any that do not begin with "p=."
Please note: I suspect that one of your answer choices was p(b) = $10.50p. If that's the case, please make sure to include the "(b)" part, as p(b) denotes "p is a function of b."
The independent variable here is b, the number of mats; the dependent var. is p, the cost of b mats. Thus, p = f(b).