Restrictions to prove quasiconcavity utility function

Question

I have the utility function given as $$u(x)=x_1+x_2^\gamma.$$ The answer says that for a utility function to exhibit strict quasi-concavity, $\gamma$ needs to be between $0$ and $1$; but I don't understand why this is true. I know strict quasi-concavity means that if $f(x)\ge f(x′)$, then $f((1−\lambda)x+\lambda x′)>f(x′$) but I don't know how this can be shown in this example...