What is the cost of one roll of plain wrapping paper and one roll of holiday wrapping paper?

3p + 4h = 123
12p + 2h = 240multiply by -2

3p + 4h = 123
-24p - 4h = - 480 (result of multiplying by -2)
add
-21p = - 357
p = -357/-21
p = 17 < plain wrapping paper (p) is $ 17 per roll

3p + 4h = 123
3(17) + 4h = 123
51 + 4h = 123
4h = 123 - 51
4h = 72
h = 72/4
h = 18 <== holiday wrapping paper (h) is $ 18 per roll

Cd is the perpendicular bisector of both xy and st, and cy=20.find xy