There are other ways of looking at a problem like this to figure it out.
For example, the problem tells you that the total (# of cars) is 1400. The word "total" is a dead giveaway that this is the right side of the equation. You can draw this out as a long rectangle and place the 1400 inside.
The left side of the equation will be parts of that rectangle. You know in this problem that there are 2 parts (east-west and north-south). Draw another long rectangle underneath the "total" one, but add a line to divide it into two unequal parts. You know the parts are unequal because one is 3/4 of the other. Label the larger portion of the rectangle X. Label the other portion of the rectangle 3/4 X.
Sometimes a diagram is more helpful than laying the problem out in words. It helps you visualize the relationships.