Answer
The structures are shown in the attached image. The explanation is below.
Work Step by Step
In isopropyl cyclohexane, the isopropyl group can rotate so that the hydrogen is near the axial hydrogens on the carbons 3 and 5. This is similar to a methyl group's hydrogen, so therefore it is similar to a methyl group's energy.
But, the tert-butyl group points a methyl group towards the hydrogens on carbons 3 and 5, which causes diaxial interactions. So the energy of this conformer jumps dramatically.