You will need to complete a Punnett square to answer this. Let's say tongue rolling is R and non-rollers are r. The heterozygous roller would be Rr and the non-roller would be rr.
Rr x rr
That yields two offspring with Rr (rollers) and two rr (non-rollers). The ratio is 2:2 or 50%. See the attached image for the completed Punnett square.