At first i was really confused when i saw this problem, i had to think for almost 30 mins before i realised that the answer just looking at me korokoro I was really disappointed that the problem took me some time for figure out though and I thought it was an interesting problem so i decided to share. See the problem below

nlfpmod
Mynd44
farano
OAM4J
dominique

5(4)-1/2(2)4-1/2(2)(5)-1/2(1)(4)=20-11=9

The solution lies in subtracting the area of the greyed out triangle ABC from the square.
That gives us three other triangles.
The ones formed by :
AC leftsided downwards pointing -
Base = 2 units, height = 4 units.
0.5 x b x h gives area.
0.5 x 2 x 4 = 4 sq. units.
[hr/]
Rectangle is 4units by 5units so is 20sq units in area.

CB righsided downwards pointing -
Base = 2, height = 5
Area = 0.5 x b x h = 0.5 x 2 x 5 = 5 sq units

AB right pointing lower triangle -
Base is 4, height is 1
Area is 0.5xbxh = 0.5x4x1 = 2 square units.

Area of ABC = Area of rectangle - (AC+CB+AB)
Area of ABC = 20sq units - (4+5+2)sq unit
Area of ABC = 9 sq units.