Can someone explain cos and sin? !

Using sine and cosine, it's possible to describe any (x,y) point as an alternative, (r,θ) point, where r is the length of a segment from (0,0) to the point and θ is the angle between that segment and the x-axis. This is called the polar coordinate system, and the conversion rule is (x,y) = (rcos(θ),rsin(θ )). 

$7.50x+$4.50y=midk it's been awhile since i've done one of these

Not sure what the question is?