Answer:
x = 6
Step-by-step explanation:
first find the angle
51 + 72 + x = 180
123 + x = 180
x = 180 - 123
x = 57
so the third angle is 57
now find the value of x in the given expression
8x + 9 = 57
8x = 57 - 9
8x = 48
x = 48/8
x = 6
now check whether the value of x is correct or not by evaluating the value of x in expression
8x + 9 = 57
8(6) + 9 = 57
48 + 9 = 57
57 = 57
so its true
Method 2
51 + 72 + (8x + 9) = 180
123 + 8x + 9 = 180
132 + 8x = 180
8x = 180-132
8x = 48
x = 48/8
x = 6
now again you can check by evaluating the value of x in the above expression
