Question

y^2=25+x
-x+5y=25

Answer 1

replace x in the first equation by 5y-25

Answer 2

that is solve the second equation for x to get
\[-x+5y=25\]
\[5y-25=x\] then the first equation gives
\[y^2=25 +5y-25\]

Answer 3

you get
\[y^2=5y\]
\[y^2-5y=0\]
\[y(y-5)=0\]
\[y=0\] or
\[y=5\]