So you know that all the angles in a triangle must add up to 180. Therefore, you can make this equation:
[tex](-20x+3)+(-40x+18)+(-15x+9)=180[/tex]
And now just solve for x!
[tex]-75x+30=180\\-75x=160\\x=\frac{160}{-75}\\x=-2.4[/tex]
So, your answer is:
x = -2.4
