Answer:
100 = 7y/cos x + 3x/cos y 
Explanation:
In the picture attached, the question is completed. 
The length of the lights is the addition of the 2 hypotenuses (h1 and h2). 
cos x = 7y/h1 
Isolating h1: h1 = 7y/cos x 
 cos y = 3x/h2 
Isolating h2: h2 = 3x/cos y 
Adding the lengths of the hypotenuses and substituting:
100 = h1 + h2 
100 = 7y/cos x + 3x/cos y