(You only need one constant of
integration).
This is called the "general solution" of the differential
equation.
We can determine C if we were given one point on the graph
of the function y(x).
For instance, if you were given that  y(1)=2 ,  then you could
substitute 2 for y and 1 for x and get:
and so you would conclude that C = -2, so the solution of the
initial-value problem:
is                                ,     or (better):