Answer:
$36.69 
 
Explanation:
Okay, so let's first write the equation: 
 
2.59*10 + 3d = 135.97 
 
Now, let's work on isolating d by first simplifying the equation: 
 
25.9 + 3d = 135.97 
 
25.9 + 3d - 25.9 = 135.97 - 25.9 
 
3d = 110.07 
 
3d/3 = 110.07/3 
 
d = 36.69 
 
Okay, now let's check: 
 
2.59*10 + 3*36.69 = 135.97 
 
25.9 + 110.07 = 135.97 
 
135.97 = 135.97 
 
Okay, so so it costs $36.69 per pair of shoe.