Let Sn denote sum of first n terms.
∴ Sn=n2(a+l), where a is the first term and l is the last term.
As given, sum of first ten terms is 210
⟹ S10=102(a+l)
⟹ 210=5(a+l)
⟹ 42=a+l ............ (1)
And also l−a=36 ............ (2)
Solving (1) and (2) simultaneously we get
l=39
Substituting l in (2) we get, a=3.
Hence, 3 is the first term.