SPEED      TIME       DISTANCE
A               r          t           2800
B               r+50       t-3         2000

Let "u" be the speed of the faster plane (plane B).
Then the speed of the plane A is (u-50) km/h.

The literal translation of the condition is this equation:

 = 3.

Indeed,  is the time for the plane A, while  is the time for plane B.

To solve the equation, multiply both sides by u*(u-50). You will get

2800u - 2800(u-50) = 3u*(u-50),  or

3u^2 - 150u - 140000 = 0.

Solve this quadratic equation using the quadratic formula.