In order to determine the value of y, first find the value of x.
Take into account that the sum of the interior angles of a triangle is equal to 180 degrees.
Then, you have:
120° + 20° + x = 180°
Simplify left side of the equation and solve for y, as follow:
140° + x = 180°
x = 180° - 140°
x = 40°
Then, the value of x is 40°
Now, consider that angle x and angle y are supplementary, that is, they add up 180°. Then, you ahve:
x + y = 180°
Replace the value of x and solve for y, as follow:
40° + y = 180°
y = 180° - 40°
y = 140°
Hence, the value of y is 140°