Integration in \(\mathbb{R}^2\) (boundary depending on \(x\))

Represented is the set \(M:=\{(x,y)^\top\in \mathbb{R}^2\,: \, a\leqslant x \leqslant b,\; \underline{y}(x)\leqslant y \leqslant \bar{y}_u(x)\}\)

\(a=\)
\(b=\)
\(\underline{y}(x)=\)
\(\overline{y}(x)=\)