To solve the problem, we need to use the relation between the angles to find a relation involving x. Then we need to solve for x and use that value to determine the measure of the desired angle.
The diagram shows you ...
... ∠KIJ + ∠JIH = ∠KIH
The problem statement tells you ∠KIJ = ∠JIH, so we can substitute ∠JIH in the above equation to get
... ∠JIH + ∠JIH = ∠KIH
... (4x+3) + (4x+3) = (12x)
... 8x +6 = 12x
... 6 = 4x . . . . . . . subtract 8x
Since we want to know the value of 12x, we can go directly there
... 18 = 12x . . . . . . multiply the above equation by 3
m∠KIH = 18°