First we would find the area of the entire thing,
A=1/2(base)(height)
so A = 1/2(5)(12) = 30 mm^2
next we find the area of the smaller triangle,
A = 1/2(4)(3) = 6 mm^2
Then we subtract the area of the smaller triangle from the area of the larger one
30-6 = 24 mm^2
