Q. let the cost of a donut be $d and that of a latte be $l.

    according to question

                     2d + 3l =6.15 -------------------(1)

                     3d + 4l =8.35 --------------------(2)

           now solving the two equations for d and l we get

         multiply 3 on both sides of equation (1) and 2 in equation (2) then eliminating d from both    the equations,

            l =18.45 - 16.7 = 1.75

       => l = $1.75 hence d =$0.45