Cannot develop the correct formula to solve for X

[mrstoked]

[h=2]I'm trying to write a formula to solve for X in the image below. I want to know how many 5-star reveiws a client would need to get to an average of 4.5 stars if they already have 6 reviews averaging 4 stars.

Any suggestions? It's driving me crazy because I was never too good at algebra.[/h]

 

[RoryA]

x=(6*(4.5-4))/(5-4.5)

substitute the relevant cell references.