If 3 + k, 18 - k, 5k + 1 are in A.P. then find k

If 3 + k, 18 - k, 5k + 1 are in A.P. then find k
Solution :
If a, b, c are in A.P then 2b = a + c
a = 3 + k, b = 18 - k and c = 5k + 1
2(18 - k)  = 3 + k + 5k + 1
2(18 - k)  = 6k + 4
18 - k = 3k + 2
18 - 2  = 3k + k
4k  = 16
k = 4

Hence the value of k is 4.