You can use the same trick to solve all of these problems. As a hint, use the fact that
If a ≤ b, then for any n ≥ 1, na ≤ nb.
As an example, here's how you could approach the first of these: If n ≥ 1, then 2n + 3 ≤ 2n + 3n = 5n. Therefore, if you take n0 = 1 and c = 5, you have that for any n ≥ n0 that 2n + 3 ≤ 5n. Therefore, 2n + 3 = O(n).
Try using a similar approach to solve the other problems. For the second problem, you might want to use it twice - once to upper-bound 5n + 1 with some linear function, and once more to upper bound that linear function with some quadratic function.
Hope this helps!