Let's set the population of Zimbabwe in 1950 equal to x.
So in 2003, the population was 12.6 million. So our equation should equal 12.6 million. Here's how you turn the other parts of the problem into an equation:
4x + 1m = 12.6 m
Now to solve.
4x = 12.6m - 1m
4x = 11.6m
x = 11.6m/4
x = 2.9m
In 1950, the population was 2.9 million