x - jedna kratka
[tex]6x+3x+3x = 18cm\\12x = 18cm\\x = 1,5cm \\Obw F_{2} = 2*(2*1,5) + 2*(3*1,5) = 2*3 + 2*4,5 = 6 + 9 = 15 cm \\[/tex]
