How to include both Fixed and unknown values in fsolve

I am wanting to solve a system of equations for 2 unknowns and 1 "fixed" value. I have read through Matt's Answer here but am even more confused on how to do so. Here is the code. x and y are my unknowns and u is a "fixed" value.

r=[1 2];
u= 5;
guess=[1,1];
output= fsolve(myfun(r,u),guess);

 function F=myfun(r,u)
  x = r(1);
  y = r(2);

F(1)=x+5*y^2+u;
F(2)=x^2+4*y+u;
end

Cheers,