Since supplement angles mean that their values add together to equal 180˚, we can use this information to solve for x, and then solve for the angle measure of angle DTJ.
(angle DTJ) + (supplement of angle DTJ) = 180
Since the question states that the supplement of angle DTJ is equal to 4/5(measure of angle DTJ), we substitute that in:
(angle DTJ) + 4/5(angle DTJ) = 180
Since we know the value of the measure of angle DTJ is 20 (1/7x + 2), we substitute that in.
20 (1/7x + 2) + 4/5 (20 (1/7x + 2)) = 180
I'm not exactly sure if you meant (1/7)x or (1/(7x)), but I assumed you meant the first (however, if you meant the latter, feel free to let me know and I'll solve that problem for you!)
Now, I will distribute the 20 to the ((1/7)x + 2) and the 4/5 to the 20 ((1/7)x + 2):
(20/7)x + 40 + 16 ((1/7)x + 2) = 180
Now, I will distribute the 16 to the ((1/7)x + 2):
(20/7)x + 40 + (16/7)x + 32 = 180
Now, I will add like terms, namely (20/7)x with (16/7)x and 40 with 32:
(20/7)x + (16/7)x + 40 + 32 = 180
(36/7)x + 72 = 180
Now, I will start to isolate the x by subtracting 72 from both sides.
(36/7)x + 72 - 72 = 180 - 72
(36/7)x = 108
Now, I will isolate x by multiplying both sides by 7/36, since it will cancel out the 36/7 on the left side.
(7/36) * (36/7)x = 108 * (7/36)
x = 21
Therefore, since the angle measure of DTJ is given as 20 ((1/7)x + 2), we substitute x = 21 in.
angle DAG = 20 ((1/7)x + 2) = 20 ((1/7)(21) + 2) = 20 (3 + 2) = 20 (5) = 100˚