Question 1015850
The problem needs to be broken into steps - 

15 gallons is 3700*15 cc or 55500 cc

The length of the trough is 3.5m or 350 cm. 

We divide {{{55500 cc/350 cm}}} and get a cross section of 158.57 sq cm. 

Next, see how the height cuts the triangle into 2 new triangles that form a square? 

*[illustration Screen_shot_2016_01_26_at_9.21.36_AM.png]

Since we have the cross section, the area of that triangle, it's just the square root of the number 158.57 or 12.593 cm