Let $g^1\colon\mathbb{R}\to\mathbb{R}$ and $g^2\colon\mathbb{R}\to\mathbb{R}$ be concave functions, and let $f\colon\mathbb{R}\to\mathbb{R}$ be a non-decreasing function (i.e., $f(x)≥f(y)$ whenever $x≥y$).
Let $h\colon\mathbb{R}^2\to\mathbb{R}$ be defined by: $$h(x_1,x_2)=f(g^1(x_1)+g^2(x_2)).$$
How do I prove that $h$ is quasi-concave?