(8m)*((210 cm + 350 cm)/2)*(d cm)
This should look familiar, albeit a bit odd for the units.
1) Draw the cross-sectional trapezoid.
2) Construct perpendiculars to the top, through the point of intersection of the base and the sides.
3) Contemplate the triangles on the two ends. You should have created two.
4) The base of these new triangles is (350 - 210) / 2. Do you see why?
5) Notice that as you play with the water level, you get lovely similar trianlges on the two ends.
This should lead you to a solution.