Question

If x2=100 then x2-x=?

Answer 1

X² = 100
x² -x = ?
As x² = 100 = 10², it means x =10
So,  x² - x = 100 - x = 90
90 is the answer.

Answer 2

For this case we have the following expression:
 $x^2=100$
 From here we clear x, obtaining two possible results:
 $x = +/- \sqrt{100}$
 $x1=+\sqrt{100}=10$
 $x2=-\sqrt{100}=-10$
 Therefore we must evaluate the second expression for both values of x:
 
 For x = 10:
 $x^2-x=(10)^2-10 x^2-x= 100-10 x^2-x=90$

 For x = -10
 $x^2-x=(-10)^2-(-10) x^2-x= 100+10 x^2-x=110$