-12<=3x-7<=-2 is two inequalities combined into one statement.
One inequality is -12<=3x-7.
The other inequality is 3x-7<=-2.
You don't say what you are trying to do with them, but if you are trying to "solve for x", you can do it separately or all at once.
For instance if you had 5 > 2x+3 > -1 you could do it separately...
5 > 2x+3
5-3 > 2x
2 > 2x
2/2 > 2x/2
1 > x
and
2x+3 > -1
2x > -1-3
2x > -4
2x/2 > -4/2
x > -2
then combine them getting 1 > x > -2. It's a little nicer to write it as -2 < x < 1.
Or you can do it all at once.
5 > 2x+3 > -1
5-3 > 2x+3-3 > -1-3
2 > 2x > -4
2/2 > 2x/2 > -4/2
1 > x > -2 which can be written as -2 < x < 1.
You work with inequalities using exactly the same rules as working with equations EXCEPT when multiplying or dividing by a negative number you must change the sense of the inequality. For instance...
-2x < 8
-2x/(-2) > 8/(-2) :::Note the change of sense.
x > -4
