Dora drove east at a constant rate of 75 kph. One hour later, Tim started driving on the same road at a constant rate of 90 kph. For how long was Tim driving, before he caught up to Dora? 
I have tried to use a table
i set it up like this
Name|D(mi)|R(kph)|T(hr.)|
Dora  | d     |75      |t      |
Tim   |d       |90      |t+1  |
and then I went and i tried to put it into an equation
75t=90(t+2)
75t=90t+180
-90t -90t
-15t=180
____ ____
-15 -15
t=-12
but It is not an answer on my multiple choice and it wouldn't be a negative 12 hours