Term for an AP = a + (n-1)d.
The third term = a + (3 -1)d = a + 2d = 21
The eight term = a + (8 - 1)d = a + 7d = 56
a + 2d = 21
-
a + 7d = 56
__________
0 -5d = -35
-5d = -35
d = -35/-5
d = 7
Substitute d = 7 in a + 2d = 21
a + 2d = 21
a + 2*7 = 21
a + 14 = 21
a = 21 - 14
a = 7
The first term, a = 7.
