Differentiation — practice question

The curve $C_1$ is given by the equation $y = x^2 - 4x + 7$. The curve $C_2$ is given by the equation $y^2 = 4x + k$, with $k$ constant. The tangent to $C_1$ at the point where $x = 3$ is also tangent to $C_2$ at the point $P$.