Answer
$CXXIX$
Work Step by Step
RECALL:
$C=100
\\L=50
\\X=10
\\V=5
\\I=1$
When a smaller number is written before a bigger number, the smaller number is subtracted to the bigger number.
Note that:
$129 = 100 + 20 + 9
\\129 = 100 + 10 + 10 + (10-1)
\\129=CXXIX$
