SAT Lines, angles, and triangles Question

A property owner wants to find the length $a$ , in feet, across a pond as represented in the figure. $\overline{PR}$ intersects $\overline{ST}$ at point $Q$ , and $\angle PTQ$ is congruent to $\angle RSQ$ . The lengths represented by $PQ$ , $TQ$ , $QS$ , and $RS$ were determined to be 4,200 feet, 8,400 feet, 1,400 feet, and 1,600 feet, respectively. What is the value of $x$ ?