I want check my answer
Q 1 : If the second term = 6 and fifteenth = 25 of sequence find the first term ?
No, because you haven't answered the question fully. You've been asked to find the first term a but you found the common difference d.
Your working seems fine though. Simply sub your value of d into one of your original equations to find a.
(eq1)
(eq2)
Rearrange eq1:
(eq3)
sub eq3 into eq2