I assume this is an arithmetic sequence
28 = a₁+6d
19 = a₁+3d
9 = 3d
d = 3 therefore, the common difference is 3
Solve for first term
28 = a₁+6(3)
28 = a₁+18
a₁ = 10
Therefore, the nth term rule for this arithmetic sequence is
an = 10+3(n-1)
an = 10+3n-3
an = 3n+7
