a)
[tex]a = 18 \ cm\\b = 5 \ cm\\c = 10 \ cm\\\\P_{b} = 2ac+2bc= 2c(a+b) = 2\cdot10\cdot(18+5) = 20\cdot23 \ [cm^{2}]\\\\\boxed{P_{b} = 460 \ cm^{2}}[/tex]
[tex]P_{c} = 2P_{p}+P_{b}\\\\P_{p} = a\cdot b = 18\cdot5 = 90 \ [cm^{2}\\\\P_{c} = 2\cdot90+460 \ \ [cm^{2}]\\\\\boxed{P_{c} = 640 \ cm^{2}}[/tex]
b)
[tex]a = 2,6 \ cm\\b = 2,6 \ cm\\c = 35 \ mm = 3,5 \ cm\\\\P_{b} = 2ac+2bc = 2c(a+b) = 2\cdot3,5\cdot(2,6+2,6) \ \ [cm^{2}]\\\\P_{b}=7\cdot5,2 \ \ [cm^{2}\\\\\boxed{P_{b} = 36,4 \ cm^{2}}[/tex]
[tex]P_{c} = 2P_{p} + P_{b}\\\\P_{p} = a^{2} = 2,6^{2} = 2,6\cdot2,6 = 6,76 \ [cm^{2}]\\\\P_{c} = 2\cdot6,76 + 36,4 \ \ [cm^{2}]\\\\P_{c} = 13,52 + 36,4 \ \ [cm^{2}]\\\\\boxed{P_{c} = 49,92 \ cm^{2}}[/tex]