A rectangular pen is made with 100 m of fencing in three sides. The fourth is a stone wall. Find the greatest possible area of such an enclosure.

Subsititue the values inthe area formula which is

A = L (100-2L) = 100L-2L^2

after that you'll need to maximize this quadratic. Maximus and minimus of quadratics occur at the vertex. FInd the vertex and the y-coordinate is the maximum area