The given differential equation is linear with constant coefficients,
[tex]\dfrac{d^2y}{dx^2} - 49y = 0[/tex]
with characteristic equation
[tex]r^2 - 49 = 0[/tex]
and hence characteristic roots [tex]r=\pm7[/tex]. This means the general solution to the ODE is
[tex]y = C_1 e^{7x} + C_2 e^{-7x}[/tex]
In fact, you're given the solution already,
[tex]y = C_1 e^{kx} + C_2 e^{-kx}[/tex]
and you've determined that
[tex]\dfrac{d^2y}{dx^2} = k^2 (C_1 e^{kx} + C_2 e^{-kx}) = k^2 y[/tex]
Comparing this to the given ODE, it's obvious that [tex]k=7[/tex], so you can just replace [tex]k[/tex] with 7 in the given template solution.