If x^2 - 9 = 0 and x > 0, which of the following must be equal to 0 ?
Choices.
A) x^2 - 9x
B) x^2 - 9x + 20
C) x^2 - 2x + 3
D) x^2 + 2x - 3
E) x^2 - 5x + 6
x^2-9 = 0   x= +3 or x = -3   if we substitute x= 3 in the equation x^2 -5x+6 = 9-15+6= 
<gadget id="calculator">9-15+6</gadget>
<output>0</output>
0   x^2 - 5x + 6 is also the answer
<result>E</result>
If x^2 - 9 = 0 and x > 0, which of the following must be equal to 0 ?