Solving a sequence question simultaneously

Hi

Question: I have to find the common difference and the first term when T8= 21 and T20= 105.

What I have done is state the rule : a + (n-1)d

Then came up with the equations a= 19x + 105 and a= 7x + 21

In y = mx +c form:

Then I obtained the common difference: 19d + 105 = 7d+ 21

19d +84 = 7d

12d+84

d = 7

And then I tried to get to get the first term by subtituting 7 into equation however this did not work:

I got the common difference correct but not the first term, can someone explain what I did incorrectly.

Help is greately respected and appreciated

Thank you