Are you permitted to use an ISA chart for this question?
If so then it will include columns for the TAS at LSS, and the relative density at various altitudes.
Just go down the LSS column multiplying the TAS by .76 to get the TAS for Mach0.76 at each altitude.
IAS = TAS x square root of relative desnity. So just go down the relative density column multpilying 300 KIAS by the square root of relative density to get the TAS at 300 KIAS.
Now sketch two lines to represent the values calulated above. The two lines will cross at the required altitude.
From a quick check using an ISA chart provided for JAR exams I make it 26000 feet from