Under & overbracket

\[
\underbrace{u'-P(x)u^2-Q(x)u-R(x)}_{\text{=0, since~$u$ is a particular solution.}}
\]

\[
\underbracket{u'-P(x)u^2-Q(x)u-R(x)}_{\text{=0, since~$u$ is a particular solution.}}
\]

\[
\overbrace{a+b+c}^{d} \quad \overbracket{a+b+c}^{d} \quad
\underbrace{a+b+c}_{d} \quad \underbracket{a+b+c}_{d}
\]