TRY
long roundedResult = Math.round(rental)

Show us  the code.
Math.round(yourNumber) returns a long if yourNumber is a double - but
returns an int if yourNumber is a float.
If you're wanting to round to a certain number of places, there are several
approaches.
1.  Look at the NumberFormat class in the API -
OR
2. to round to 2 digits - multiply by 100 - then round to an int, then
divide by 100.

To round a float or double, use Math.round or add .5 and convert to
int.

To round a BigDecmial use BigDecmial.round