this is an expression, meaning it doesn't have an equal sign. therefore it cannot be solved, only reduced into its simplest form. to do so, you must change both fractions into a common denominator. in this case its 12. so you have 4/12 and 15/12, or 1 3/12. Together, this adds to be 1 and 7/12.
your answer in simplest form is 1 7/12 + a