No, that's not the right answer. 

Be sure you're starting with Newton's Law of Cooling,

T_surf = T_o + T e ^{- t/}

75^oC = 25^oC + (70^oC)e ^{- 5 min/}

Find the "time constant" and then find the time t that gives T_surf = 35^oC.

(c) 2000, Doug Davis; all rights reserved.