A = x·(350 - 2x)
Therefore we get quadratic function:
A(x) = -2x² + 350x
where x = h is one of the dimensions and k = A(h) is the largest area

250-2x = 350 - 2·87.5 = 350 - 175 = 175 m

The largest area that can be enclosed is 45762.5 m²
The dimensions of the enclosed area are 87.5m and 175 m