The answer to Problem 1 is the number -1. You use PEMDAS to evaluate division first, then you subtract later.
For problem 2, we combine like terms to get this answer: -4a+b. You only add the 'a' terms together, same with the 'b' terms. Think of it like having 2 books and add on 3 more books. We can write that as 2b+3b = 5b. So we have 5 books total.
Problem 3) The answer here is 1.2n after adding n to 0.2n. Think of the first n as 1n or 1.0n
Problem 4) Divide both sides by 15 and you end up with the answer a = 2
Problem 5) Answer: 31.4 This answer is found by multiplying the diameter (10) by the approximate value for pi (3.14). The formula is C = pi*d