Convolution Equation Help
Think about what is happening when you do convolution.  You are taking h(m) and reflecting it about m = 0, and offsetting it by n.  You can demonstrate this:
On one piece of paper, draw x(m) = u(m). On a second piece, draw h(-m). 
Line them up at zero. This is n = 0. The two overlap only at zero. Now shift the second piece to the right by one. Now you have n = 1. They overlap at 0 and 1. Shift it right again and you have n = 2. And so on.
You should be able to see why the summation is zero for all m &lt; 0, and for all m > n