Hello, please explain how to get the formula i have highlighted , thanks !

._._ '- Production per eective worker is a function of the capital stock per e'ecve worker. With constant returns to scale, the size of the economy does not mat- ter. Countries with the same technology 3 and the same capital per worker K [N will have the same GDP per worker independent of their size. This seems reason- able. at least as a rough approximation.' To simplify notation, we use lower case I: to denote the capital stock per worker: K k= EN and we also use the simplied notation X f\"? =F(EJV' I) . Note that f ' (k) is the marginal product of capital We see that the marginal product of capital depends on how much capital there is per effective worker. With this notation, the steady state capital stock per effective worker. 16. is determined by the following condition