The sum of three numbers is 75. the third number is 2 times the first. the second number is 9 less than the first. what are the numbers?

Let the three numbers be x, y and z.  Then x+y+z=75.
z = 2x.  y = x - 9.

Eliminate y and z by substitution:  x + (x - 9) + (2x) = 75

Then 4x = 75 + 9, or 4x = 84.  x = 21, y = 21 - 9 = 12, and z = 2(21) = 42.

Check:  Do 21, 12 and 42 add up to 75?