Constant True Altitude MET
Two factors impact your altitude as you travel from place to place: pressure, and temperature. You need to know how both of these work.
For pressure, if you fly towards a low pressure, it means the pressure levels are bending towards the surface - and if you maintain a constant indicated altitude, you will follow this level towards the ground. The saying to remember here, therefore, is "high to low (pressure), look out below." For high pressure, the effect is the opposite.
For temperature, the effect is similar - if you fly towards somewhere with equal surface pressure but that has colder air, the pressure levels will "contract together" - meaning again that the pressure levels are bending towards the ground and, if you maintain a constant indicated altitude, you are flying towards the ground. Hence the saying "cold kills." Again, for warm air the effect is the opposite.
In this question, it states you are flying towards an area with a different QNHQNH —Static pressure at MSL calculated from QFE using ISA temperature lapse rates, but that true altitude is constant. Simply think about what effect the change in pressure would have had, and do the opposite with the change in temperature to cancel out the change made by the pressure.