the absolute value of a number is never negative, only positive or zero
examples:absval(12)=12, absval(-15)=15, absval(0)=0
therefore absval(10)=10 and absval(-10)=10 also
now you know where the second value of -10 comes from
simply speaking, the absolute value of a number is the number of units that number is from the origin without regard to direction
in your example, x is the number that is 10 units from the origin; there are two numbers that are each 10 units from the origin on the number line: 10 and -10(don't forget that these are undirected distances-which direction you move does not matter, in the positive direction or in the negative direction) I hope this helps you understand absolute value.