In this case, you would be solving for K.
K + 13 = 2 + 17
- 13 - 13
K = -13 + 2 + 17
Combine all the like terms (add and subtract)
K = -13 + 2 + 17
K = -11 + 17
K = 6
When you see questions like this, you would want to get the variable (K) by itself.