Let's first simplify the left side of the equation:

10 - x(5 - 6 - x)
10 - x(-1 - x)
10 + x + x^2

Next, let's simplify the right side of the equation:

x(x + 3) - 4x
x^2 + 3x - 4x
x^2 - x

Now we can set the simplified left side equal to the simplified right side:

10 + x + x^2 = x^2 - x

Rearranging the terms, we get:

10 + 2x = 0

Subtracting 10 from both sides gives us:

2x = -10

Dividing by 2 gives the solution:

x = -5

Therefore, the solution to the equation is x = -5.