`a) 3^{x}.2^{x + 1} >= 72`
`-> 3^{x}.2^{x}.2 >= 72`
`-> 3^{x}.2^{x} >= 36`
`-> 6^{x} >= 6^{2}`
`-> x >= 2`
`-> x in [2; +infty)`
`b) D = RR \\ {1}`
$(\dfrac{1}{2})^{\dfrac{1}{x - 1}} < (\dfrac{1}{2})^{4}$
`-> 1/(x - 1) < 4`
`-> 4x - 4 > 1`
`-> 4x > 5`
`-> x > 5/4`
`-> x in (5/4; +infty)`