Let's find the intercepts of the line:
x-intercept (when y=0):
73ax+b0=4
73ax=4
x=283a
y-intercept (when x=0):
73a0+by=4
by=4
y=4b
The triangle has vertices at (0,0), (283a,0), and (0,4b).
Area of the triangle =21×base×height
=21×283a×4b
=21×1123ab
=563ab square units