five times a number, subtracted from eighteen, is triple the number.
let the number be x
Five times the number is 5x
subtract this from 18 and you get 18-5x
This is equal to tripple the number tripple x is 3x
so
18-5x=3x
add 5x to both sides
18 = 8x
divide both sides by 8
18/8 = x
x = 9/4
x = 2.25