How many days would it take for a crepe myrtle tree to grow 1 cm in height if it grows 3.38 feet per year?

It's very easy, it's depending mainly on converting a mathematical unit:

When 1 Cm = 0.0328084 Ft
So, we assume that: 
in    1 year → it grows 3.38 Ft

So  X(years)→ 0.0328084

∴X(years) = 0.0328084 * 3.38 = 0.0097 year

We have to convert this unit form years to days by:

       1 year = 365 days
So,  0.0097 =  X (days)
∴X(days)= 365 * 0.0097 = 3.5 days

So it will take 3.5 days to grow up

; lc