Distributing a Subtraction

Multiply every term inside the bracket by the number outside, and keep the sign of each term. So $3\left(x-4\right) = 3x-12$, because $3\cdot \left(-4\right) = -12$. If you write $3x-4$, the second term has not been multiplied. To solve $3\left(x-4\right) = 9$, expand the bracket, add 12 to both sides, then divide by 3:

$$3\left(x-4\right) = 9$$

$$3x-12 = 9$$

$$3x = 21$$

$$x = 7$$

A negative number outside a bracket changes every sign inside: $-2\left(x-5\right) = -2x+10$. Read the outside number with its sign, and multiply each term inside by it. Check the answer above: $3\left(7-4\right) = 3\cdot 3 = 9$.