Let x be time in seconds and y1 and y2 be the height of the planes at x seconds.
The equation for Plane A is given by,
y1 = 30.5x + 2652 -----------------(1)
The equation for Plane B is
y2 = 85.75x -----------(2)
The planes meet when y1=y2, so
85.75x = 30.5x +2652
or 85.75x - 30.5x = 2652
or 55.25x = 2652
or x = 2652/55.25
or x = 48
Now, putting the value of x in eq(2) we get:
y2 = 85.75(48) = 4116 feet
After 48 seconds both plane will be at 4116 feet of height