Let Susan be X
Let Alice be Y

Susan = Alice - $120
X = Y - $120 (Equation 1)


6*(Alice - $210) = Susan
6Y - $1260 = X (Equation 2)


Sub equation 1 into equation 2.

6Y - $1260 = (Y - $120)
5Y = $1140
Y = $228 (Therefore Alice has $228)

Susan has $228-$120 = $108

OK!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!