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Perimetro di base $\small 2p= 2(3,5+2,5) = 2×6 = 12\,cm;$
area di base $\small Ab= 3,5×2 = 7\,cm^2;$
altezza del prisma $\small h= \dfrac{V}{Ab} = \dfrac{42}{7} = 6\,cm;$
area laterale $\small Al= 2p×h = 12×6 = 72\,cm^2;$
area totale $\small At= Al+2Ab = 72+2×7 = 72+14 = 86\,cm^2.$