X^2 +15x +3?

I think you need "=0" on the end of that equation but I'll take it as implied. The general way to solve a quadratic equation is to imagine the factor of x² as a, x as b, and the numeric part as c, then use the equation:

 ̣· ̣· ̣· -b ± √ (b² - 4ac)
x = ----------------------
̣̣· ̣· ̣· ̣· ̣· ̣· ̣· ̣· 2a

a=1, b=15, c=3

So x is:

-15 ± √(225-12)
----------------------
̣· ̣· ̣· ̣· ̣· ̣· 2

or:

-15 ± 14.594519519326424197332749726524
------------------------------------------------------------------
̣· ̣· ̣· ̣· ̣· ̣· ̣· ̣· ̣· ̣· ̣· ̣· ̣· ̣· ̣· ̣· ̣· ̣· ̣· 2

So:
-0.2027402403367879013336251367379

And:
-14.797259759663212098666374863262