A company's revenue, R⁡(x) , in thousands of dollars, from selling x thousand units is modeled by the quadratic function shown. R⁡(x)=−x^2+15⁢x−28 Its cost, C⁡(x) , in thousands of dollars, is modeled by the linear function C⁡(x)=4⁢x How many units must the company sell to break even?