Inscribe triangle RST in the square with dimensions 4×4, as shown in the figure.
from the area of this square, 4*4=16, we remove the triangles with dimensions
3×4, 2×1 and 2×4, whose side lengths are shown in the figure, and we are left with the area of triangle RST.
so

units squared
similarly,

units squared
Thus, the areas are equal.