bantuin mengerjakan INI dong plis!!

the answer for number 7 is b

no 9. c). Rp.2000,00
caranya :
buku = x
pensil = y

Eliminasi :
2x+y = 4750 |×2|
5x+2y = 11250 |×1 |
---------------------
4x+2y = 9500
5x+2y = 11250
-------------------- -
-x = -1750
x = 1750

Mencari nilai y :
2x+y = 4750
2(1750)+ y = 4750
3500+y = 4750
y = 1250

chandra membeli :
x+y =1750+1250=3000
Uang kembalian :
5000-3000 = Rp.2000

no 7. d). x^2-18x+77 = 0