There is few ways to get the result. The easiest is calculating step by step (year by year) price of the computer.
50%=0.5 (in decimal)
21000.5=1050 - 1 year
10500.5=525 - 2 year
The result it 2 years.
To get more accurate result we can solve equation
\(2100*( \frac{1}{2})^x=600\)
but its very hard to solve without powerful tool.
