Answer:
d = 4
Step-by-step explanation:
a7 = -9
a19 = 39
We know the formula for an arithmetic sequence is
an = a1+d(n-1)
Using the givens:
a7 = -9 = a1 +d(7-1)
a19 = 39 =a1 +d(19-1)
Subtracting these equations
-9 = a1 +d(6)
39 =a1 +d(18)
------------------------
-48 = 0 -12 d
Divide by -12
4 =d
