what is the difference between [-3] and -3?

Question

I thing [-3]is double?

Answer 1

I assume you mean |-3|

the | | stands for absolute value, that is, we always take the positive value of whatever is inside the two bars.
so |-3| = 3 and of course |3| = 3

so |5-9| = 4 , and
|9-5| = 4

so if you have an equation such as |2x - 7| = 5
we have two solutions,
2x-7 = 5 or -(2x-7) = 5
2x = 12 or -2x + 7 = 5
x = 6 or x = 1

check, you will see either value works in the original equation