First step you would have to do would be to times 35 by 12 to see how much you would have to pay for 12 months.
35 x 12 = 420
Then, as you said, you a down payment of 600 so then you'd add.
600 + 420 = 1,020.
So the total cost of the computer would be $1,020.
So what you can do is take 49 and divide it by 2 cause times is multiplication so if you take 49 and divide that by 2 you would get 24.5. Now if you plug so like 24.5 · 2 would would get 49.
~Good Luck~
F(x) = 0.5x + 10
0.5x + 10 = 18.5
0.5x = 18.5 - 10
0.5x = 8.5
x = 8.5/0.5
x = 17 days
That's is depend from the author of the equation. But most time there is need to multiply value X into constant before.
In other words it look likes
y=2*x-3