To start, we know that an isosceles trapezoid means that the length of P to Q is equal to the length of S to R. Therefore, taking that P is at (0,0), we can say that the distance formula from P to Q is equal to the distance formula from R to S. Our formulas are then 
if we take that g is the x value for R and b is the y value for R, seeing that Q and R are on the same Y value. Therefore, if we square both sides, we figure out that a²=(g-c)² and a=√(g-c)², so a is either equal to g-c or c-g. As the x value of R is clearly lower than the X value of S, and a is positive, we can take that a=c-g. Thus, because we want to solve for g, or the x value of R, we can add g to both sides to get g+a=c. Then, we can subtract a from both sides to get g=c-a, so you can put a as your answer.
Feel free to ask further questions, and Happy Holidays!
