Subtraction doesn't mix with mult. or div. Are you sure you've copied this problem down correctly?
"decrease an amount (x) by 19" would be x - 19 (NO multiplication here)
3y^2 and -3/4y^2
5x and 2/3x
12x^2
One way to do this is to notice

Then

We have

and since
has a period of
,

and so
