When you're looking for the volume of a cylinder, you have to use the equation 
And this V is equal to volume, R is equal to radius, and H is equal to height.
In this case, you've got to find what r²is. To do this, square 5. 5x5= 25.
You've then got to substitute this into the equation, along side H, which is equal to 7, and this gives you V=π*25*7, you've then got to multiply these all together (using π as 3.142) this gives you 549.85cm³
Therefore, you need 549.85cm³ of water to fill the pipe.
Hope this helps :)
And this V is equal to volume, R is equal to radius, and H is equal to height.
In this case, you've got to find what r²is. To do this, square 5. 5x5= 25.
You've then got to substitute this into the equation, along side H, which is equal to 7, and this gives you V=π*25*7, you've then got to multiply these all together (using π as 3.142) this gives you 549.85cm³
Therefore, you need 549.85cm³ of water to fill the pipe.
Hope this helps :)