PLEASE HELP ILL GIVE BRAINLIEST

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