Hello from MrBillDoesMath! 
 
Answer: 
Positive x-intercept = + sqrt(3)
Negative x-intercept = - sqrt(3) 
Discussion: 
Generally, the x-intercept of a function g(x) occurs where g(x) = 0. In our case this is
g(x) = 0 =>
3x^2 - 9 = 0 => (add 9 to both sides of the equation)
3 x^2 = 9 => (divide both sides by 3)
x^2 = 9/3 = 3 =>
 
x = +\- sqrt (3)
Positive x-intercept = + sqrt(3)
Negative x-intercept = - sqrt(3)
Thank you, 
 
MrB