When you graph these two equations on a graphing calc, you should be able to see one dotted line and one solid, as well as the solutions being mostly in the 1st quadrant.
15+c=17.50
First you subtract 15 by 15 because you switch operations from adding to subtracting then you would put C under it
Second subtract 17.50 and 15 and that is 2.50 so C= 2.50
