Can you help me with the while loop?

Missing initialization seems to be the issue. You need to initialize 'FinalValue' as 0 to remove the error as well as 'Rate' and 'Investment' to the suitable value.

Rate = 5; % use your desired value
Investment = 1000; % use your desired value
numYears = 0;
FinalValue = 0;
while (FinalValue <= 3*Investment)
    numYears = numYears+1;
    FinalValue = Investment*power(1+Rate, numYears);
end
% disp(FinalValue);
disp(numYears);

This code works but it displays the FinalValue to be greater than 3*Investment as anyhow the loop will run once, since initially the condition FinalValue <= 3*Investment is true. To achieve this condition, you may use an if statement within the loop to calculate previous year when your desired condition held.

Rate = 2; % use your desired value
Investment = 1000; % use your desired value
numYears = 0;
FinalValue = 0;
while (FinalValue <= 3*Investment)
    numYears=numYears+1;
    FinalValue=Investment*power(1+Rate,numYears);
    if FinalValue > 3*Investment
        numYears = numYears - 1;
        break
    end
end
% disp(FinalValue);
disp(numYears);