Let $f(x)=2 x+1, x_{1}=0, x_{2}=2, x_{3}=4, x_{4}=6,$ and
$\Delta x=2$
(a) Find $\sum_{i=1}^{4} f\left(x_{i}\right) \Delta x$
(b) The sum in part (a) approximates a definite integral by 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.