Suppose $X$ has a $\rm{Binomial}(n,p)$ distribution. Then its moment generating function is
$\begin{align} M(t) &= \sum_{x=0}^x e^{xt}{n \choose x}p^x(1-p)^{n-x} \\ &=\sum_{x=0}^{n} {n \choose x}(pe^t)^x(1-p)^{n-x} \\ &=(pe^t+1-p)^n \end{align}$
Can someone please explain how the sum is obtained from lines (2) to (3)?