Sorry, let me restate the original problem and be more explicit.

Find the convolution of x(n) and h(n).

where

where the convolution is

note the

symbol is not multiply, it is the convolution operator. Also y(n) is causal, which means y(n) is only defined for

, where n=0,1,2,3....

Now,

Plugging in,

Simplifying,

The argument passed into the unit step must be

or else the value of the unit step function is 0. So if I need to change my limits of summation so that the two unit step functions resolve to 1. That is,

if

then

.

if

then

.

So the limits on k would be,

if

if

I am bamboozled by the two lower limits on k. How can I do a summation with two lower limits? Would the summation diverge?