the 3rd and 7th terms of an arithmetic progression are 6 and 30 respectively. determine the

(I) common difference
(ii) first term
(iii) 10th term

just use your basic formulas....

a+2d=6, a+6d=30
subtract them:
4d=24
d = 6, then mentally ... a = -6
10th term = a+9d
= -6+54
= 48