Question 777963
z=x^2-yx
subtract z from each side
x^2-yx-z=0 now use the quadratic formula
for equation in the form ax^2+bx+c=0
{{{x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}} 
so we have
{{{x = (y +- sqrt( (-y)^2-4*1*(-z) ))/(2*1) }}} 
{{{x = (y +- sqrt( y^2+4z ))/(2) }}}