3 times the quantity 2 decreased by x is 9

3 times the quantity 2 decreased by x is 9

The quantity 2 decreased by x. The phrase decreased by means we subtract:
2 - x

3 times the quantity:
3(2 - x)

The word is means equal to, so we set 3(2 - x) equal to 9:
3(2 - x) = 9