complimentary angles add to equal 90°. Write two equations to solve for two variable.
J + K = 90
J = K - 18
use substitution or elimination to solve 
I will use substitution. substitute (K - 18) for J in the first equation to combine equations into one in only variable K. 
(K - 18) + K = 90
2K - 18 = 90
2K = 90 + 18
2K = 108
K = 108/2
K = 54° 
J and K are complimentary. 
J + 54° = 90° 
J = 36°