Question

find the value of x.

Explanation:

Step1: Find the interior - angle adjacent to 128°

The sum of an interior - angle and its adjacent exterior - angle of a triangle is 180°. Let the interior - angle adjacent to 128° be $y$. Then $y + 128^{\circ}=180^{\circ}$, so $y = 180^{\circ}-128^{\circ}=52^{\circ}$.

Step2: Use the angle - sum property of a triangle

The sum of the interior angles of a triangle is 180°. In the triangle, we know two interior angles: 78° and 52°. Let the third angle be $x$. Then $x+78^{\circ}+52^{\circ}=180^{\circ}$.

Step3: Solve for $x$

$x=180^{\circ}-(78^{\circ}+52^{\circ})=180^{\circ}-130^{\circ}=50^{\circ}$.

Answer:

$50$