Find the expression for the cdf $\displaystyle F_x(x)$

We were actually given the solutions:

I understand the first part, concerning where x<0 but I don't understand when $\displaystyle x\geq 0$; why do we take the limits and equation from when x<0 + the limits and equation for $\displaystyle x\geq 0$

Please explain this to me. Thanks.