lets make the cost of the algebra textbook = x and the cost of the geometry book = y
2x+3y=64
4x+2y=66
now we get one of these equations in terms of one variable
2x+3y=64 ---> 2x=64-3y ---> x=32-1.5y
now we can plug that numeric value back into the other equation
4(32-1.5y)+2y=66
we can now simplify this equation in order to get the value of y
4(32-1.5y)+2y=66 ---> 128-6y+2y=66 ---> 128-4y=66 ---> -4y=-64 ---> y=16
