1961 AHSME Problems/Problem 3

Problem

If the graphs of $2y+x+3=0$ and $3y+ax+2=0$ are to meet at right angles, the value of a is:

The slope of the first graph is -1/2. The slope of the second is 2, since it is perpendicular, and it is also -a/3 by rearranging. Thus a=-6. These problems are copyrighted © by the Mathematical Association of America, as part of the American Mathematics Competitions.