If 3=3, 4=15 and 6=51 then 8=?

Question

If
2,6=5
4,9=7
6,11=8
8,15=11
then
12,18=?

Answer 1

After the = sign is written like Roman numbers
3 = 3 = III
4 = 15 = IV
6 = 51 = VI
thus 8 = VIII = 53

Answer 2

53 is the answer
3=3=III
4=15=IV=15
6=51=VI=51

8=VIII=53