Calculating how many more shares to create

Question

1. Shares in company: $3000$

2. Valuation: $\$3$ million

3. Person $A$ $2000$ Shares ($\$2$m in equity)

4. Person $B$ $1000$ Shares ($\$1$m in equity)

If person $C$ came along with $\$1$m and wanted to invest it in the company, what formula would you use to calculate how many more shares should be created to give person $C$ a fair stake in the company. I know that the answer is $1000$ more shares need to be created as that's what person $C$ is entitled to (to give them a $25\%$ stake in the company) - But for the life of me I can't figure out a formula.

What I've got so far is:

$\frac{1m}{1m+3m} = 0.25$ - What their fair share in the company should be after investing.

$0.25 = \frac{x}{3000 + x}$ - $x$ is the number of shares that need to be created, but I can't resolve for $x$ because $x$ is there twice.

Thanks, sorry if this question is dumb.