If I am reducing the lower limit of a summation how does this work algebraically

I have this as a solution and don't know where it comes from

Summation from x=1 to infinity of (1/2*exp(t))^x

The lower limit is then reduced;

1/2*exp(t) * Summation from x=0 to infinity of (1/2*exp(t))^x

Where does the 1/2*exp(t) come from ??

I thought it was x=0 for f(x) added to the expression but that is simply 1