If the vertices of a triangle are (1,-3), (4,p), and (-9,7) and its area is 15 sq. units, find the value(s) of p.

The vertices of the triangle are A(1,-3), B(4,p), and C(-9,7).
Area of triangle = 15 sq. units.
Area of the triangle with vertices (x1,y1); (x2,y2); (x3,y3) is 1/2[x1(y2-y3) + x2(y3-y1) + x3(y1-y2)]
Therefore, 1/2[1(p-7) + 4(7+3) -9(-3-p)] = |15|
p-7 + 40 + 27 + 9p = 2 x |15|
10p + 60 = ±30
10p = -30 or -90
p = -3 or -9