Answer
$1990$
Work Step by Step
RECALL:
$M=1000
\\C=100
\\X=10$
Note that when a smaller numeral is written before a bigger numeral, the value of the smaller is subtracted to the value of the bigger numeral.
Thus,
$MCMXC = 1000+(1000-100)+(100-10) = 1000+900+90=1990$
