Answer:
2
Step-by-step explanation:
an = a1 + d(n-1) is the formula
17 = a1 +d(8-1 ) = a1 *7d
39 = a1 +d (19-1) =a1+ 18d
We have 2 equations and 2 unknown. Subtract to eliminate a1
39 = a1 +18d
-17 = a1 +7d
--------------------
22 = 11d
Divide by 2
22/11 = 11d/11
2 =d
The common difference is 2
