X=7
Explanation:
A triangle equals 180°, and they gave us one length already, and it says the one arm of the triangle is equal to the other so we know that the third angle is going to be the same as the second angle.
180=90+(6x+3)+(6x+3)
Combine like terms
180=96+12x
Subtract 96 from both sides
84=12x
Divide 12 by both sides
7=x
Now check it,
90+(6*7+3)+(6*7+3)
90+(45)+(45) =180