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. Solve the equation for a. If the atmospheric temperature is −45.8 °F and the ground temperature is 20 °F, what is the altitude?