$P_{x}=15600\left(1+{3\cdot 0{,}075\cdot (1-0{,}13) \over 12}\right)^{18 \over 3}(1-0)^{18 \over 12}$

{\displaystyle P_{x}=15600\left(1+{3\cdot 0{,}075\cdot (1-0{,}13) \over 12}\right)^{18 \over 3}(1-0)^{18 \over 12}}