plug in the numbers given into the formula for volume of a cylinder (V = pi r^2 h). 26 is the diameter of the cylinder, making 13 the radius, which you have put. the height is 40m.
the equation should now look like this:
V = pi 13^2 (40)
Solve from left to right because of PEMDAS
After simplifying, you should get 21237.16634 m^3 (volume is cubed)