An arch in shape of the semi-ellipse at its bottom extremity is to support the walkway crossing over the highway. Create the coordinate system with positive y-axis projecting vertically upward from center of highway and x-axis crossing perpendicular to highway at ground level, positive axis to right. Highway itself is 80 feet wide. Ends of arches are 100 feet apart. Clearance at edges of highway should be 13 feet from highway vertically to bottom of arch structure. If equation of ellipse which will meet the specifications is written in form x^2/(a^2) + y^2/(b^2) = 1, y > 0.