To show that
\begin{equation*}
\lap{f'(t)} = s\lap{f(t)} - f(0), \quad s \gt 0\text{,}
\end{equation*}
we start with the definition of the Laplace transform:
\begin{equation*}
\lap{ f'(t) } = \lim_{b \to \infty} \ub{\int_0^b e^{-st} \cdot f'(t)\ dt}_{I}\text{.}
\end{equation*}
Applying integration by parts to \(I\) once in \(\lap{ f'(t) }\) with
\begin{align*}
u = e^{-st}, \quad\amp dv = f'(t)\ dt, \\
du = -se^{-st}\ dt, \quad\amp v = f(t)
\end{align*}
integration by parts gives
\begin{align*}
\lap{ t^{n} }
\amp = \lim_{b \to \infty} \left[f(t)e^{-sb}\Bigg|_0^b - \int_0^b f(t)\cdot\left(-se^{-st}\right)\ dt\right]\\
\amp = \lim_{b \to \infty} \left[\left(f(b)e^{-sb} - f(0)e^{0}\right) + s\int_0^b e^{-st}\cdot f(t)\ dt\right]\\
\amp = \lim_{b \to \infty} \left[f(b)e^{-sb} - f(0) + s\int_0^b e^{-st}\cdot f(t)\ dt\right]\\
\amp = \lim_{b \to \infty} \left[f(b)e^{-sb}\right] - f(0) + s\lim_{b \to \infty} \left[\int_0^b e^{-st}\cdot f(t)\ dt\right]\\
\amp = \lim_{b \to \infty} \left[f(b)e^{-sb}\right] - f(0) + s\ub{\int_\infty^b e^{-st}\cdot f(t)\ dt}_{\large\lap{f(t)}}
\end{align*}
Recall that a requirement for the Laplace transform of \(f(t)\) to exist is that
\begin{equation*}
\lim_{b \to \infty} \left[f(b)e^{-sb}\right] = 0, \quad s \gt 0\text{.}
\end{equation*}
Thus, setting the limit to zero gives us the desired result
\begin{equation*}
\lap{ f'(t) } = s\lap{f(t)} - f(0)\text{.}
\end{equation*}