The given equations can be written as a matrix:
color(white)("XXXX")xcolor(white)("X")ycolor(white)("X")=cXXXXxXyX=c
color(white)("XXX")((color(red)(1),color(blue)(2),,color(green)(9)),(color(red)(1),color(blue)(1),,color(green)(1)))
If D is the determinant formed by the x and y columns of this matrix:
color(white)("XXX")D=|(color(red)(1),color(blue)(2)),(color(red)(1),color(blue)(1))| = color(red)(1)xxcolor(blue)(1) - color(red)(1)xxcolor(blue)(2) = -1
and D_x is the determinant of the same submatrix but with the x column replaced with the c column
color(white)("XXX")D_x=|(color(green)(9),color(blue)(2)),(color(green)(1),color(blue)(1))|=color(green)(9)xxcolor(blue)(1)-color(green)(1)xxcolor(blue)(2)=7
and D_y is the determinant fo the same submatrix as D but with the y column replaced with the c column
color(white)("XXX")D_y=|(color(red)(1),color(green)(9)),(color(red)(1),color(green)(1))|=color(red)(1)xxcolor(green)(1)-color(red)(1)xxcolor(green)(9)=-8
Cramer's Rule tells us
color(white)("XXX")x= (D_x)/(D) = 7/(-1) = -7
and
color(white)("XXX")y=(D_y)/(D) = (-8)/(-1) = 8
