first represent the unknowns using variables:
let x = number of 24-cent stamps used
let 2x = number of 39-cent stamps used (according to the second statement)
since the total is 7.14 dollars,
0.24x + 0.39(2x) = 7.14
now we solve for x:
0.24x + 0.78x = 7.14
1.02x = 7.14
x = 7 (number of 24-cent stamps used)
hope this helps~ :)