XV is already in Roman numerals. It translates to 15 in westernised Arabic numerals, i.e. our usual notation.
XV = 15 __ XV = 15000 (A bar over a number means to multiply it by 1000) __ XVM = 16000
roman numarals are v,i ,x,c and some other stuff with represent numbers
XII
in roman X
Your answer is XXI
L
538
CCCLXII
1 = I
XII
DCC
CXXVI