Why can’t the answer be -2 and -18? We know |x| = +x and -x. Don’t we get two answers for x?
The absolute value of a number always returns its positive value. So |4| = 4, and |-4| = 4. In other words, |x| = +x. I think the confusion may lie in the case where we have an equation, where we have something like:
|x| = 2
This means that x = +/-x, or:
x = 2, -2
However, this property doesn't come into effect in this question—here, we are just evaluating the absolute value of -4, which is 4. If this doesn't seem familiar, I'd recommend reviewing the lesson on absolute values.