One way to approch this problem is consider what you would see as a person standing on the side of the road. Let's say you are holding a stopwatch and you hit start as the faster car (we will call its velocity v_{1}) passes you. As defined in the problem, at this instant, the slower car (v_{2}) is 121m away (let's call this x_{0} for generality). If we were to watch the cars, intuitively the faster car will pass the slower one, but it may not be obvious when that would happen. Instead, let's translate this event of one car passing the other into math. The faster car passes the slower one simply translates to both cars being in the same location, we will use x_{1} and x_{2} to represent the position of the faster and slower cars respectively. In other words we consider the cars passing when x_{1} = x_{2}. Now because the cars are moving, we need to figure out how the position of each car changes in time. The faster car starts next to you and moves away at velocity v_{1}. Therefore its position in time is described as x_{1} = v_{1} * t. The slower car starts a distance x_{0} away and moves away with velocity v_{2}. Therefore its position is described by x_{2} = v_{2} * t + x_{0}. Now we have 3 expressions:

1) x_{1} = x_{2}

2) x_{1} = v_{1} * t

3) x_{2} = v_{2} * t + x_{0}

With these expressions, we can determine the time at which the cars pass.

In general, the approach I took to this problem was to:

1) Define (mathematically) the criteria, in this case the criteria: x_{1} = x_{2}

2) Describe the variables in the equality (x_{1}, x_{2}) in terms of the constants given (v_{1}, v_{2}, x_{0}) and the variable we are interested in solving for (t)

3) Plug in and solve algebraically for the variable (t)