AMC 10 2021-fall-a Problem 11

Emily sees a ship traveling at a constant speed along a straight section of a river. She walks parallel to the riverbank at a uniform rate faster than the ship. She counts $210$ equal steps walking from the back of the ship to the front. Walking in the opposite direction, she counts $42$ steps of the same size from the front of the ship to the back. In terms of Emily's equal steps, what is the length of the ship? $\textbf{(A) }70 \textbf{(B) }84 \textbf{(C) }98 \textbf{(D) }105 \textbf{(E) }126$