I this equation correct?

If |m| = - 18 then m = - 18 and m = 18

No. |m| can only be positive.

Step-by-step explanation:

Absolute value |m| indicates magnitude, i. e. it has no "direction". m=-18 indicates that m is 18 units left (negative) of position 0; it has direction. |m| asks for the value of m without any direction, i. e. its magnitude.

If it was: |m|=18 then the rest would be true.

See also magnitude vs vectors.