In this case, we'll have to carry out several steps to find the solution.
Step 01:
Data
∠ M = 90
∠ K = 78
KL = 35 ft
MK = ?
Step 02:
cos α = adjacent / hypotenuse
α = 78
adjacent = MK
hypotenuse = KL = 35
cos 78 = MK / 35
35 * cos 78 = MK
7.277 = MK
The answer is:
MK = 7.3 ft