Use "implicit differentiation" to find the slope of tangent when x= sqrt 3 for x^2 + y^2 = 4

i did

dx^2/dx + dy^2/dx = d4/dx

dx^2/dx = 2x

dy^2/dx = 2yy'

d4/dx = 0

2x + 2yy' = 0

y' = - x/y

and then what do i have to do to find the slope of the tangent ?