this is based on the commutative law of multiplication and the associative law of multiplication.
the commutative law says that a * b = b * a
the associative law says that a * (b * c) = (a * b) * c
x = 3 * -x + 68 becomes x = 3 * -1 * x + 68 which becomes x = -1 * 3 * x + 68 which becomes x = (-1 * 3) * x + 68 which becomes x = -3 * x + 68 which becomes x = -3x + 68.
here's a reference to the different laws / properties of arithmetic operations.
