I'm not sure if my title makes any sense... sorry, it has been a while since I've done cal.

I'm taking a Probability course and I'm reading one of my profs solutions that involves finding the moment-generating function of a continuous random variable. During the integration steps I got lost. Can someone please help understand how he gets from one step to another (I'll just include the part that confuses me):

[snip]

Q2: How does he get from the second line to the third? What method is that? [snip]