a.
`663851 : x = 897`
`⇔x = 663851 : 897`
`⇔x = 663851/897`
b.
`9187 - y + 409 = 89^2 - 102`
`⇔9187 - y + 409 = 7921 - 102`
`⇔9187 - y = 7921 - 102 - 409`
`⇔9187 - y = 7410`
`⇔y = 9187 - 7140`
`⇔y = 1777`
c.
`8*6 + 288 : (x-3)2 = 50`
`⇔48 + 288 : (x - 3)2 = 50`
`⇔288 : (x -3)2 = 2`
`⇔(x - 3)2 = 144`
`⇔x - 3 = 72`
`⇔x = 75`
d.
`48751 - (10425 + y) = 3828 : 12`
`⇔48751 - (10425 + y) = 319`
`⇔10425 + y = 48751 - 319`
`⇔10425 + y = 48432`
`⇔x = 48432 - 10425`
`⇔x = 38007`