Let f(x)=3x+6,x1=1,x2=4,x3=7,x4=10, and x=3. (a) Find i=14f(xi)x. (b) The sum in part (a) approximates a definite integral using rectangles. The height of each rectangle is given by the value of the function at the left endpoint. Write the definite integral that the sum approximates. (a) i=14f(xi)x=270 (Simplify your answer.) (b) Find the definite integral that is approximated by the sum in part a. i=14f(xi)x113(3x+6)dx