Trapezoid question- algebra/geometry?

In trapezoid y1, y2, y3, y4, Each corner is at an elevation with y1 & y2 being the top of the trapezoid and y3 & y4 being the base. In my dilemma, y1=960, y2= 960.5, y3=959, y4=967.5. The height of the trapezoid is from stations- where the base x2 (side of y3,y4 or bottom of trapezoid) is at station- 55.73 and x1 (side of y1, y2 (or top)) is at station 0.1. I am trying to figure out a formula to determine at what station (x3) would y5=6. Can you please provide a formula using x1,x2,x3 & y1,y2,y3,y4,y5- so that I can utilize the formula for other stations and elevations?thanks in advanceCorrect- my bad. I guess y6-y5=6. Where y5 & y6 would be the x3 distance from x1.