Atmospheric pressure is used to determine altitude of an airplane. the relation between atm. pressure P in...?

atmospheric pressure is used to determine altitude of an airplane. the relation between atm. pressure P in (lbs./sq.in) and altitude is given by the equation of the form P= a(b^x), where altitude x is measured in miles. If P= 14.7 psi at sea level what is the value of a? Find b, if P = 7.53 at an alt. of 3 miles.