1)

AD = AF - FD

AD = 6 - x

2)

Aire(ABCD) = L × l

Aire(ABCD) = 4 × (6 - x)

Aire(ABCD) = 24 - 4x

3)

Aire(FDC) = (L × l)/2

Aire(FDC) = (4 × x)/2

Aire(FDC) = (2 × 2 × x)/2

Aire(FDC) = 2x

4) On se demande quand 24 - 4x = 2x

24 = 2x + 4x

24 = 6x

24/6 = x

4 = x