Explanation
Step 1
check every interval
[tex](-\infty,-7\rbrack[/tex]
means all values smaller or equal than -7
in math terms
[tex]x\leq-7[/tex]
Step 2
[tex]\lbrack9,\infty)[/tex]
it means all numbers greater or equal than 9,in math terms
[tex]x\ge9[/tex]
Step 3
finally
[tex]\cup\text{ = or}[/tex]
then, the solution is
[tex]x\leq-7\text{ or x}\ge9[/tex]
I hope
