Two large telecommunications companies are considering a merger. The net income, in billions of dollars, of the first company can be modeled by the function A(t) = 9.01t ^ 2 - 20.60t + 23.55 , where t is the number of years since 2013. The second company's net income, also in billions of dollars, can be modeled by the function B(t) = 9.47t ^ 2 - 21.46t + 18.72 , where is the number of years since 2013. The function C(t) can be used to model the two companies' combined net income, in billions of dollars, where t is the number of years since 2013. Write the function c(t) .