Every time you see r, replace it with -3. Every time you see s, replace it with 5. You are working wth exponents, which means that the little number next to the big number (exponent) is how many times you multiply the big number by itself.
For example, 5^3=5x5x5=125 (isn't one of the problems you are trying to solve)
If the expodent is negative, then you divide. Sso if it's like 5^-3, you do 1 ÷ 5 ÷ 5 ÷ 5 = 0.008
Hope this helps! :)