$x-\text{pomysˊlana dodatnia liczba},x>0$   

 

$\left(\frac{x^2+3x}{x}\cdot 100-300\right)x=\left(\left(x+3\right)\cdot 100-300\right)x=100x\cdot x=100x^2$  

$100x^2=4900$  

$x^2=49$  

$x=7\vee x=-7<0$  

$\text{Pomysˊlana liczba to 7.}$