Finding the lower and upper Riemann Sums

Question

I'm having a hard time figuring this questions out. I've looked on google and on the book and so far I haven't gotten a good explanation for this questions. I know they aren't hard and are probably easy but I've haven't gotten a good explanation.

1. Find the lower and Upper Riemann Sums $L(P)$ and $U(P)$ for the function $f(x)=x^2$ on the interval $[0,1]$ using the partition $P=\{0,\frac12, \frac34,1\}$

2. Given that $\int_1^3 f(x)~dx=4$ and $\int_1^5 f(x)~dx=7$, find $\int_3^5 f(x)~dx$.

3. Given that $\int_1^3 f(x)~dx=4$ and $\int_1^3 g(x)~dx=2$, find $\int_1^3 3f(x)-g(x)~dx$.