Question 11984
You first need to "translate" the english equation to an mathematical equation.
You should get something that looks like this:
{{{20-X=42}}}
You want the X by itself, so you MUST substract 20 from both sides
{{{-X=22}}}
Since -X=22, then that means X=-22

*CHECK*
{{{20-(-22)=42}}}
If you subtract a negative, you a basically adding it. 
{{{20+22=42}}}
{{{42=42}}}
Since the last equation is true, your answer is true.
Hope this helps!