No that's as low as it goes
20000 x 90/100 = 18000
18000 x 90/100 = 16200
The value of the car after 2 years is $16200
If it's addition you do the opposite which is subtraction and that's the answer that you get so 6m_2=m+13 so add 6-2=4 and then 13-4=9