The economic order quantity

Total cost.
$$C(q)=\underbrace{\frac{KD}q}_{\text{ordering}}+\underbrace{\frac{hq}2}_{\text{storage}}=\frac{60\,000}q+1.5\,q,\qquad q>0.$$

Minimum. $C'(q)=-\frac{60\,000}{q^2}+1.5=0\iff q^2=40\,000\iff q=200$. Since $C\to+\infty$ at $0^+$ and at $+\infty$ and $q=200$ is the only critical point, it is the global minimum (equivalently, $C'<0$ on $(0,200)$ and $C'>0$ on $(200,\infty)$).

Answer. Order $200$ items at a time ($6$ orders a year). The cost is $C(200)=300+300=600$ € per year.

General formula. $q^*=\sqrt{\dfrac{2KD}h}$ (the Wilson formula). Here $\sqrt{2\cdot50\cdot1200/3}=200$.

Remark. Again, ordering cost and storage cost are equal at the optimum. Treating $q$ as a real number is harmless here, because the optimum happens to be an integer.