Question 1013975
That is ONE equation with TWO variables, so if that is all, then nothing to "solve".  Your "try to find the y axis" makes no sense.  The y-axis is the y-axis;  the {{{y=25}}} is not clear.  


Best you could do is to look for x and y intercepts, and with whatever skills you know up to now, find axis of symmetry, and the vertex.


Here is what you can do.
{{{-1*x^2-10x=y=0}}}
{{{x^2+10x=0}}}
{{{x(x+10)=0}}}
meaning the x-axis intercepts are 0 and -10; which you can also call the "roots" of {{{-x^2-10x=0}}}.


The form of the equation {{{y=kx^2+something}}} tells you that you have a vertical axis of symmetry; and it will occur in the exact middle between the x-axis intercepts, as YOU ALREADY FOUND, {{{x=-5}}} to be this symmetry axis.


The vertex happens where {{{x=-5}}}, so you find the y-coordinate using {{{y=-x^2-10x}}}
{{{(-5)^2-10*(-5)}}}
{{{25-(-50)}}}
{{{25+50}}}
{{{75}}}
The vertex is  (-5,75), and this is a MINIMUM, based on, again, the form of the equation you have.