Can you get the side?

The total blue area is $100^2 + 120^2 + 80^2 = 30800$ . Then half the blue area is $15400$ . Let the length to be found be $x$ . We note that the difference in height between the square B and square C is $120-80 = 40$ . Then we have:

$\begin{aligned} (100+120)(40+x) + 80 x & = 15400 \\ 22(40+x) + 8x & = 1540 \\ 30x & = 1540 - 880 = 660 \\ \implies x & = \frac {660}{30} = \boxed{22} \end{aligned}$

The area of the whole figure is 100 100 + 120 120 + 80*80 = 10000 + 14400 + 6400 = 30800cm^2 Divide that by 2 because of the line gives us 30800cm^2 / 2 = 15400. Focus on the top half and imagine that there is a rectangle on top of square C to make the top half a perfect rectangle like this

The rectangle is (120-80)(80) = 40 * 80 = 3200cm^2. Add that to the top half of the figure and get 15400 + 3200 = 18 600cm^2 The length of the perfect rectangle is 100 + 120 + 80 = 300 cm Find the breadth of the perfect rectangle and get 18600cm^2 / 300cm = 62 cm. We have found the length in the question mark plus the breadth of the imaginary rectangle. Take 62cm - 40cm = 22 cm