ANSWER :
a. 12 + x = 24
EXPLANATION :
Substitute x = 12 to the following choices and check if the equation is true.
12 + x = 24
12 + 12 = 24
24 = 24
True
15 + x = 15
15 + 12 = 15
27 = 15
False
x + 12 = 30
12 + 12 = 30
24 = 30
False
x + 5 = 18
12 + 5 = 18
17 = 18
False
The only option that is true is 12 + x = 24
