The standard way is THIS:


1.  Introduce new variable u = x^2.   Then your original polynomial takes the form

    q(u) = 2u^2 - 5u - 7.


2.  Find its zeroes. For it, use the quadratic formula

     =  =  =  = .


    --->   =  = ,     = -1.


3.  So, the decomposition for q(u) is

    q(u) = 2u^2 - 5u - 7 = 2(u-(7/2))*(u-(-1)) = (2u-7)*(u+1).


    Therefore, the decomposition for the original polynomial is

    p(x) = 2x^4 - 5x^2 - 7 = (2x^2-7)*(x^2+1).


    You may factor it further


    p(x) = ,   if you want.