Approximate areas under graphs using trapezoidal rule

Approximate $\int_1^3\left(4-x\right)dx$∫31​(4−x)dx by using four rectangles of equal width whose heights are the values of the function at the midpoint of each rectangle.

Approximate $\int_3^{15}\left(4x+6\right)dx$∫153​(4x+6)dx by using four rectangles of equal width whose heights are the values of the function at the:

The function $f\left(x\right)=5x$f(x)=5x is defined on the interval $\left[0,6\right]$[0,6].