For each function, evaluate the expression and interpret the result.

(a) f(3, -4), where f(x, y) = xy represents the multiplication function

(b) M(120, 5), where M(m, g) = computes the gas mileage when travelling m miles on g gallons of gasoline

(c) V(0.5, 2), where V(r, h) = r²h calculates the volume of a cylindrical barrel with radius r feet and height h feet.