Please help explain definite integrals and Riemann Sums, I am so confused

My professor has not answered any of my emails regarding definite integrals. Can anyone please explain this problem and solution?

I am not sure how to put the actual formula in here but this is what I have:

Consider the function f(x)= ((x^2/2)+7)

Calculate Rn for f(x)=((x^2/2)+7) on the interval [0,3] and write your answer as a function of n without any summation signs.

Rn=______

lim Rn=______

I know that the limit is 25.5 (found on another site) but I am so confused on how to actually solve it, the answer doesn't help.

I think Xi is 3n/n and the change in X is 3/n

But I have no idea where to go from there.

Can anyone please help explain what to do?