I have a question about these electric field strength formulas

Why is \$E\_1=\frac{F}{q}=\frac{K\frac{Q\_1 Q\_2}{x^2}}{Q\_2}\$, instead of \$=\frac{K\frac{Q\_1 Q\_2}{x^2}}{Q\_1}\$? 

\$K\$ is constant \$=9 \times 10^9\$

[![image 1](https://i.stack.imgur.com/twNqU.png)](https://i.stack.imgur.com/twNqU.png)