Show that the tangent to P: ax^2 + bx + c with gradient m has a y intercept of c - ( (m - b)^2 / 4a )

OK so far my working would be...

find dydx to find gradient of tangent

dydx = 2ax + b

y intercept means x = 0, so when x =0, 2ax + b = b

From here i am stuck.

Any ideas would be appreciated