Perfect thing for the first question !
For the decreasing : a function decreases when its derivative is negative.
Thus, you have to solve for .
A quick way to do is to say that it's negative if and only if the signs for 3x and 4-x are different, that is to say that if 3x is positive, then 4-x has to be negative, and if 3x is negative, 4-x has to be positive.
Hence the solutions are :
This should give you the set of values for x that you want