Tn = [a + (n-1)d]

Sn = n/2*[2a +(n-1)d].

Given

T30/T10 = 3

(a + 29d)/(a + 9d) = 3

After simplification we get

a = d

S6 = 6/2 [2a + 5d] = 42 (given)

14 = 2a + 5d

14/7 = 2 = a = d ( substituting a = d)

So the series starts like

2, 4, 6, 8, 10, 12, 14, 16, .......

Now T13 = [2+12d] = [2 + 24] = 26.