you need an x in the equation
-3* 1/6 x - 18 = 72
add 18 to each side
-3 * 1/6 x = 90
multiply 3 * 1/6
-1/2 x = 90
multiply by -2 on each side
-2 * (-1/2) x = -2 * 90
x = -180
