The third term of an arithmetic sequence is 21 and the eighth term is 56. the first term is

The first term = 7

given:
a3 = 21, a8 = 56

write as coordinates
(3,21),(8,56)

Since the sequence is arithmetic, the common difference (d) is also the slope, so you can find the value of (d) by using slope formula and treating the two givens as points

(56-21)/(8-3) = 35/5 = 7
So, d = 7 and slope = 7

Now, since we know that a3 = 21, and we need to find the value of the number two spaces behind it, we can subtract the common difference twice from a3 (which is 21):
21-7-7 = 21-14 = 7