Ok so basically we know that all the angles in a triangle add up to 180 degrees.
That means
(x-7) + (2x) + (x+11) = 180 because we combined all the angle measures.
To solve this equation we've got to combine like terms.
We can combine the x's together so we have 4x + (-7) + 11 =180.
Then we can combine -7 and 11 to get 4 because 11-7=4.
Now our equation is 4x+4=180.
We can subtract 4 from both sides of the equation to get 4x=176.
Then simplify this by doing 176/4=44.
That means x=44.