I am given the impulse response
$$h[n]=2\left (\frac{1}{2} \right )^n\sigma [n]-\delta [n]$$
where $\sigma$ is the unit step function and $\delta$ is the unit impulse function. I know that
$$a[n]=\sum_{k=-\infty}^{\infty}h[k]\sigma [n-k]=\sum_{k=-\infty}^{n}h[k]$$
Thus, my step response should be
$$a[n]=\sum_{k=-\infty}^{n} \left( 2 \left (\frac{1}{2} \right )^k \sigma [k]-\delta [k] \right)$$
How do I calculate this?