I have to find the width of a river which is represented by a right trianle. The opposite side is labeled w and the adjacent side is 1oo ft. Theta is 54 degrees.

Would my equation be opp = adjacent/cos(54)?

From what I am reading into the diagram, you have a distance down the riverbank of 100ft, then w across the river. The angle to the end of w from the start of the 100ft length is 54deg

tangent54=w/100
w= 100 tangent54.