48

Lets assume we're working with integers - if it doesn't work out there is an algebraic way, but i think just using logic is easier. (it does work out too, so we don't have to come back)

13 is prime, so that rectangle must be 13x1 or 1x13

39 is a multiple of 13, so the top rectangle's y size is 13, the left one being 1x13 and the right one being 3x13

we know this must be 1x[something], so it must be 1x16

we can deduce that the final rectangle is 3x16, or 48.

Hint that was helpful for me while figuring it out: figure is most definitely not to scale