Hi kelsikels,

Let v be the speed of the airplane, let w be the speed of the wind (absolute value).

General equation x = vt

In our case,

700 = (v - w) * 4 = 4v - 4w (Eqn 1)

700 = (v + w) * 2 = 2v + 2w (Eqn 2)

Multiply (Eqn 2) by -2

-1400 = -4v - 4w (Eqn 3)

Add (Eqn 1) and (Eqn 3)

-700 = -8w

w = 700/8 = 87.5 miles/hour