c = 14.45 (pretty sure)
You are dealing with 2 sides: a side opposite to the angle and one adjacent to the angle.
Therefore, you will be using tan to solve this, as tan(angle) = opposite side/adjacent side
Substitute tan(60 degrees) into calculator get about 1.73
Then set up an equation: 1.73 (tan) = 25/c
Multiply both sides by c and you get 1.73c = 25
Next divide both sides by 1.73 to get c
25/1.73 = about 14.45