The error is in the second step they subtracted 3 from both sides but it was a 3x, which you can not do.
Correction.
9x+18+3x=1
12x+18=1
12x=-17
x=-17/12
<h2>
Hello!</h2>
The answer is:
The expression for the number of sweets that Sara and Tim have now, are:

<h2>
Why?</h2>
To write the expressions for the number of sweets that Sara and Tim have now, we need to follow the next steps:
Sara starts with 24 sweets and Tim starts with 24 sweets

Then, Sara gives Tim x Sweets

Then. Sara eats 7 of her Sweets

Then, Tim eats half of his sweets

So, the expression for the number of sweets that Sara and Tim have now, are:

Have a nice day!