x-3(x+5)=-(3x+20)

$$\\-3(x+5)=-3x+-15=-3x-15\\\\
-(3x+20)=-1(3x+20)=-3x+-20=-3x-20\\\\
so\\
x-3(x+5)=-(3x+20)\\
x-3x-15=-3x-20\\
-2x-15=-3x-20$$

And you can take it from there.

Notice that if you multiply a bracket by a negative number, everything inside the bracket changes sign. 

x - 3(x +5) = -(3x + 20)      let's simplify this first

x - 3x - 15  = -3x - 20         combine like terms

-2x - 15  = -3x  - 20            add 3x to both sides  and add 15 to both sides

x = -5