Question

A camper wants to know the width of a river. From point A, he walks downstream 100 feet to point B and sights a canoe across the river. It is determined that = 42°. About how wide is the river?

Answer 1

Let C be the point on the other side of the river so that AC is perpendicular to AB, so we want AC.
You have a right-angled triangle, but you don't state which angle is 42°
I will assume it is at B

so tan 42° = AC/100
AC = 100tan42° = ....