East 3 mph after 30 minutes is 1.5 miles (Let it be a)

North 2 mph after 30 minutes is 1 mile (Let it be b)

the triangle formed with sides = 1.5 & 1 with the hypotenuse (c) = sqrt(1.5^{2} + 1^{2}) = 1.803

a^{2} +b^{2} = c^{2} Taking the derivative, we have 2a da + 2b db = 2c dc (dc is the unknown), then

2(1.5)(3) + 2(1)(2) = 2(1.803) dc

then dc = 3.606 mph