when using multiplying by 1 to find an expression equivalent to 18/7 with a denominator of 28x, the equivalent expression that I get is 4x/28x and i am being told that that's wrong

Answer 1

Let's look at this:
The original value is 18/7. You want to find an equivalent value that has a denominator of 28. To get that value, you need to multiply the denominator by 4. To "multiply by 1" to not change the value, you need to also multiply the numerator by 4.
So:
(18 x 4)/(7 x 4) = ?/28