So….the difference between 3 and 12 month is 2.1000 – 2.0100 = 0.0900 This is over (12-3=) 9 months, so it is changing by 0.0900/9 – 0.0100 per month
If we want 5 months forward, this is 2 months later than 3 month forward (5-3=2) so we assume that the 5 month rate will be different from the 3 month forward by 2 months at 0.0100 per month.
This gives a 5 month forward rate of 2.0100 + 2*0.0100 = 2.0300