Question 8510
You've got {{{ (3x - 1)/4 = Q/28 }}} and they're asking you to find Q, the missing numerator.


Look at both denominators. What did they multiply to 4 to get to the 28? That would be the 7. Since they used 7 to get from 4 to 28, you must also multiply the numerator (3x - 1) by 7 to be able to fill up that missing numerator on the right hand side. So, the numerator would be 7(3x - 1) which is 21x - 7.