You simply change each of the numbers to its Roman equivalent, keeping the dashes. This would be shown as VI-XXVI-MMIX.
The Roman numerals are XLV.
18 in roman numerals looks like XVIII
20 in Roman numerals is XX.
300 would be represented by 'CCC' in roman numerals.
MCMLXXXIX
DCLXVI
D1X
MCMLXXXV
MMDCCCLXXXVI
MMI
XLVI
XXI