Calculating Probabilities using a cumulative distribution function 
For (b) 
Pr(X greater than or equal to 2) = ?
The textbook says as such but I am confused:
Pr(X greater than or equal to 2) 
= 1 - pr(X less than 2)
I do not understand why they re-write the probability, because doesn't Pr(X greater than or equal to 2) fit in the domain: x greater than 0? Why must we re-write this? 
the answer then goes on to solve this
1 - F(2) 
^ I dont understand why we can substitute 2 into F(x) only after re-writing the equation as 1- F(x) and not simply F(x) 

We cannot substitute $2$ into $F(x)$ because $F(x)$ is by definition equal to $\Pr\{X\le x\}$ and we are looking for the probability $\Pr\{X\ge2\}$.
We have that
$$
\Pr\{X\ge2\}=1-\Pr\{X<2\}
$$
and
$$
\Pr\{X<2\}=\Pr\{X\le2\}
$$
since the random variable $X$ is continuous.
So the answer is
$$
1-F(2).
$$
