Question 870500
t8 = 59 = a+7d..............(1)

t21= 150 =a+20d.............(2)

solve (1) & (2)

subtract (2) from (1)

-13d =-91

d=7

a+7d=59
substitute d
a+49=59
a=10

tn = a+(n-1)d

tn = 10+(n-1)7

=10+7n-7

=7n+3