Are you working out of the blue Discovering Geometry book?
To start with, an altitude goes from one vertex to the (infinite) line of the far side at a right (perpendicular) angle.
So to find an altitude, we'll need to:
1.) Find the slope of the far side.
2.) Convert that to a perpendicular (opposite reciprocal) slope for the altitude.
3.) Take our new slope, and the vertex our altitude passes through, and make an equation of a line.
For example, if you wanted the altitude from B:
1.) The slope of the far side is
2.) The slope perpendicular to is
3.) So our new slope is , and our vertex is B(-9,-3), so our equation is: