different between y and y1

Which y1 ?

I don't see an y1.

Success!
Luc

When you solve via solve block it does not matter which variable you assign the result to.

You may use

a:=odesolve...

y:=odesolve...

or

y.1:=odesolve...

It does not matter.

But why did you encircle the definition of x.f in the photo? This just defines the end value.