Question 148473
a.  {{{x^3-4x=0}}}


First factor out x. Then you get


{{{x(x^2-4)=0}}}


Don't stop there!


Then factor {{{x^2-4}}} and get 


{{{x(x-2)(x+2)=0}}}


OK now you can stop.


b.  {{{2x^2+7x-15=0}}}


Factor the expression


{{{(2x-3)(x+5)=0}}}


You're done.