For altitudes up to 36,000 feet, the relationship between ground temperature and atmospheric temperature can be described by the formula t = −0.0035a + g, in which t is the atmospheric temperature in degrees Fahrenheit, a is the altitude, in feet, at which the atmospheric temperature is measured, and g is the ground temperature in degrees Fahrenheit. Determine the altitude in feet when t is −42.5 °F and g is 10 °F.