The relationship between marketing expenditures $(x)$ and sales $(y)$ is given by the following formula, $y=6x-0\mathrm{.}40x^2+15.($Hint: Use the Nonlinear Solver tool$).$

a$.$ What level of marketing expenditure will maximize sales?

Note: Round your answer to $2$ decimal places.

Marketing expenditure

b$.$ What is the maximum sales value?

Note: Round your answer to $2$ decimal places.

Maximum sales value