Can you help me of how to do this question, thanks ;)

$$2\frac{1}{3} \text{ is to }4\frac{1}{3}\text{ as }7 \text{ is to x}\\\\ \text{Multiply the first two numbers by 3 to get}\\\\ 7 \text{ is to } 13 \text{ as } 7 \text{ is to x}\\\\ \text{So it's now clear what x is!}$$

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sorry but can you show me in more depth I still don't get it

Turn 2 1/3 into an improper fraction.  It becomes 7/3.   

Turn 4 1/3 into an improper fraction.  It becomes 13/3.

The ratio of the two numbers is (2 1/3) divided by (4 1/3).  This ratio is the same if we multiply top and bottom by 3, to get 3*(2 1/3) divided by 3*(4 1/3).  

This is 3*(7/3) divided by 3*(13/3). 

The 3's cancel on the top, leaving 7,  and they cancel on the bottom, leaving 13. So the ratio is 7/13.  

The missing number at the start is therefore 13.

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Thank you!