Hello James! The graph only goes up to x=18 so I am unsure on how to answer for intervals extending beyond that. But part a can be answered if you mean the integral from x=0 to x=8 of f(x) dx.
We look for the areas between the curve and the x-axis from x=0 to x=8. We can interpret this as a geometry problem. We see if we break it up from x=0 to x=4 and x=4 to x=8 we have two trapezoids.
Trap 1: x=0 to x=4
(2+6)/2*4=16
Trap 2: x=4 to x=8
(2+6)/2*6=24
Thus the total integral is 40 units^2
Hope this helps!