Given, y=e4x+2e−x
Differentiating w.r.t. x
y′=4e4x−2e−x
Differentiating again w.r.t. x,
y′′=16e4x+2e−x
Differentiating again w.r.t. x,
y′′′=64e4x−2e−x
Now, y′′′−13y′−12y+7=64e4x−2e−x−13(4e4x−2e−x)−12(e4x+2e−x)+7
=64e4x−2e−x−52e4x+26e−x−12e4x−24e−x+7
⇒y′′′−13y′−12y+7=7