Solve the equation.
y + 6 = -3y + 26
Oy= -8
O y = -5
O y = 5
Oy = 8

Hi!

The area of the shaded region is
The area of the whole triangle - the area of the smaller triangle

So first let's find the area of the whole triangle. 
a = 0.5bh
a = 0.5·5·12
a = 2.5·12
a = 30

Now find the area of the smaller triangle.
a = 0.5bh
a = 0.5·3·4
a = 1.5·4
a = 6

30 - 6 = 24

The answer is 24mm

Hope this helps! :)
-Peredhel