How do i solve this question

Question

A biologist wants to know the width w of a river to properly set instruments for an experiment. From point A, the biologist walks downstream 100 feet and sights to point C (see figure). From this sighting, it is determined that 𝜃 = 54°.  How wide is the river? (Round your answer to one decimal place.)

Answer 1

The angle looking back at A and to C across the stream is 54°. There is a right triangle going from A to B to C with the hypotenuse being BC. He knows AB is 100 ft and he knows that opp/adj or AC/AB is tan(54°). He can solve for AC, the width of the river.