Let f(x)=6x24.5x\displaystyle {f{{\left({x}\right)}}}={6}{x}^{{2}}-{4.5}{x}. Find the equation of the secant line on the graph of f(x)\displaystyle {f{{\left({x}\right)}}} between x1=5\displaystyle {x}_{{{1}}}={5} and x2=12\displaystyle {x}_{{{2}}}={12}. Write your answer in mx+b\displaystyle {m}{x}+{b} format.

y=\displaystyle {y}=