Short Answer: g(x) = 4/5 x^2 - 4
The definition of a vertical stretch is a(f(x)). That means that you put brackets around everything on the right side of the equal sign and multiply by a.
g(x) = 2*f(x) = 2( (2/5)x^2 - 2)
g(x) = 4/5 x^2 - 4
Red Line: Original Equation of f(x)
Blue line: 2*f(x)
Note I have graphed the right side of the equations of f(x) and 2(f(x)
