Mary is 21 years old. Her age is 9 years more than 3 times Henry’s age. Which equation can be used to determine x, Henry’s age in years?

HELP ASAP PLEASE

A: 3x + 9 = 21
B: 21 + x = 3(9)
C: 9 + 21 = 3x
D: x + 9 = 21