Read the instructions carefully and give exact answer. Rating will be given based on quality.

Calculate the indicated Riemann sum S4 for the function f(x) = 18 -2x". Partition [0, 12] into four subintervals of equal length, and for each subinterval [Xk - 1,Xk , let ck = (2Xk- 1 + Xk) /3. S4 = (Simplify your answer.)