Question 745448
That step might be useful if you know how or are trying to complete the square.  First, look at what is simpler.  Can you factor x^2+5x-46 ?  No.  Nothing useful comes from +/- 23 or 2.  


Next is to either directly use the general solution to the quadratic formula, or to Complete The Square to solve for the variable.  This is where you got stuck with:
{{{x^2+5x-46=0 }}}
{{{x^2+5x=46}}}


The lefthand member is the same as x*(x+5).  That is like a rectangle with one side of length x and the other side of length x+5.  This rectangle contains a main square of length x, and an extra rectangular part of length 5 and height x.  There is a graphical rearrangement that can be made, which I will not provide here and will not try to describe... but what you will do to perform the symbolic process of "completing the square" is, add {{{(5/2)^2}}} to both sides of the equation.   {{{(5/2)^2=25/4}}}.


{{{x^2+5x+ 25/4=46+25/4}}}
The left side is now a square trinomial, and the righthand member is a sum of constants which can be calculated or computed.
Continuing,...


{{{(x+5/2)^2=46*4/4+25/4}}}
{{{(x+5/2)^2=209/4}}}
**{{{x+5/2=0+- sqrt(209/4)}}}
{{{x=-5/2+- sqrt(209/4)}}}
{{{highlight(x=-(5/2)+- sqrt(209)/2)}}}



**The extra 0 was added to the righthand member because without it, the rendering was given a messier wording included with the arrangement of symbols, making visual reading more difficult.