var('x,y')
y    = function('y')(x)
eqn  = diff(x^2+y^2==25,x)
sol0 = solve(eqn, diff(y,x))
sol1 = sol0[0].subs({x:3})
sol1.subs({y(x=3):4})

︠290f7f3a-98fe-4977-aa06-8a459215c444︠