Question 1172505
an=a1+(d(n-1), and 6 terms later the new value is 12 more, so d=2
so 
a7=17=a1+2(6)
a1=5
Can also figure it that if the 13 th term is 12 more than the 7th, the 7th is 12 more than the first,
a21=5+2(20)=45.