It is given that the third terms is obtained by multiplying the second term by 2 and then adding 3:
[tex]a_3 = 2a_2+3[/tex]
Since the third term is 17, we have
[tex]17=2a_2+3 \iff 2a_2=14\iff a_2=7[/tex]
And then again, the second term is obtained by multiplying the first one by 2 and then adding three:
[tex]a_2 = 7 = 2a_1+3[/tex]
Solve this equation for [tex]a_1[/tex] and you'll get the answer
