I promise you, you know very well how to do this.
Let's talk about these examples first:
If you have 10 jellybeans, and you give away 4, then how many do you have ?
I hope you said 6 . That's correct. How did you figure it out ? It's easy, right ? You just subtract (take away) 4 from 10, and you have 6 left.
If you have 8 ounces of soda in a can, and you drink 5 ounces, then how much
is left in the can ?
I hope you said 3 ounces. How did you figure it out ?
You just subtract (take away) 5 from 8, and you have 3 left.
Easy, right ?
One more quick example:
If you have a string that's 11 inches long, and you cut off 2 inches and
throw it away, then how long is the string that you have left ?
You just subtract (take away) 2 from 11 and you have 9 left.
OK. Now the real one. Don't clutch. Just relax.
If you have a board that's 46 meters long, and you cut off ' u ' meters and
throw it away, how much board do you have left ?
You just subtract (take away) 'u' from 46 and you have (46 - u) left.