For 5.1,
If t=4, write out the terms. How many are there?
If t=6, write out the terms. How many are there?
If t=8, write out the terms. How many are there?
Can you see a pattern?
So, in terms of t, how many terms are there in general?
For 5.2, try a similar thing, this time adding up the terms. Look for a pattern.