Since the sail’s shape is (approximately) a triangle, it helps to sketch a triangle representing the sail. Since we know its base width is $10$ feet, it is a good idea to label that in the sketch. The heigth is our unknown quantity, so we can use $h$ as a variable to represent the sail’s height in feet and label that too. Since the total area is known to be $125$ square feet, this suggests using the area formula for a triangle: $A=\frac{1}{2}bh\text{.}$ With this formula, we can substitute $125$ in for $A$ and $100$ in for $b\text{:}$ $$125=\frac{1}{2}(10)h$$ and this equation models the height of the pool.