Question 978217
Use the Law of Sines
a/sin a = c/sin c

There is a right triangle with the downstream leg 100 feet and angle to C 59 degrees.  
sin 59/C  is width of river
sin 31/A is 100 feet.  
This is a right triangle

sin 31/100=sin 59/x
0.5150/100=0.8572/width.
Cross-multiply
0.5150* width=85.72
width=85.72/0.5150 = 166.45 feet.