Find the number(s) b such that the average value of f(x) = 6 -42x + 70 on the interval [0, b] is equal to 15. Select the correct method. b O Set f(x) dx = 15 and solve for b 1 b O Set b f(x) dx = 15 and solve for b o 1 O Set - f(11) = 15 and solve for b O Set f(b) = 15 and solve for b b Use a comma to separate the answers as needed