Hi roberto,

If we denote x as the width of the rectangle and the side of the triangle,

then perimeter is

3x+2y=900

the area of the window is the rectangle area plus the triangle area

To maximise, we need to differentiate wrt a single variable,

so write y in terms of x

now substitute that y into the area equation,

differentiate it wrt x,

set the result =0

find the x that causes it to be zero and use it to calculate max area.