point a in the xy-coordinate system is shown below. given two other points b (4a, b) and c (2a, 5b), what is the area of triangle abc in terms of a and b?
7ab2\dfrac {7ab}{2}27ab
9ab2\dfrac {9ab}{2}29ab
15ab2\dfrac {15ab}{2}215ab
4ab4ab4ab
6ab6ab6ab