**Answer:**

Alex had been teaching 21 years.

Blake had been teaching 15 years.

Charlie had been teaching 7 years.

Alex and Blake had been teaching for a total of 36 years.

Charlie and Blake had been teaching for a total of 22 years.

Charlie and Alex had been teaching for a total of 28 years.

Let Alex = A, Blake = B and Charlie = C, then:

A + B = 36 [1]

C + B = 22 [2]

C + A = 28 [3]

If we use [3] - [2] we have:

A - B = 6 [4]

If we use [1] + [4] we have:

2A = 42

A = 21

By [1] B = 15. By [3] C = 7.