Convolution of discrete signals using convolution sum
Well, you know that $u[n]=0$ for all $n&lt;0$. So from the two terms in your sum, all terms will be zero where either $k+2 &lt; 0$ (i.e., $k &lt; -2$) or $n-2-k&lt;0$ (i.e., $k&gt;n-2$). From this your sum will need to run from $-2$ to $n-2$. Can you take it from here?