What is MMMCDLXXIV in Arabic numerals?

M = 1000

D = 500

C = 100

L = 50

X = 10

V = 1

I = 1

Add the value of the letters together unless a smaller value comes before a bigger value in which case the smaller value is subtracted:

→ MMMCDLXXIV = 1000 + 1000 + 1000 - 100 + 500 + 50 + 10 + 10 - 1 + 5 = 3474