It helps to write it out in words before trying to form the equation.
X = # cars going east-west (this is the unknown)
3/4 X = # cars going north-south
1400= total number of cars
Total number of cars = # going east-west + # going north-south
1400 = X + 3/4 X
X + 3/4 X = 1400
This is how you can set up the equation. Let me know if you need help solving the equation.