Question 56480
8x^3 - 18x

Both term have the same factor ...
8x^3 --> 2*2*2*x*x*x
18x --> 2*3*3*x
==> 2*x (the common factor)...
2x{4x^2-9}

Then the 4x^2-9 can be written in this form {2x}^2-{3}^2 --> {2x+3}{2x-3}

Therefore 
8x^3 - 18x = 2x{4x^2-9}= 2x{2x+3}{2x-3}