Answer:
d)  Mr.Wallace can use the following equations:
     a + b = 9
    2 a + 3 b = 23
Step-by-step explanation:
Here, the total number of students = 23
Number of groups = 9
Each group can have 2 or 3 students.
 a is the number of  groups of 2.
⇒ Total students in a groups  = a x (Number of  student in each group) 
= a x 2 = 2 a
 b is the number of groups of 3
⇒ Total students in b groups  = b x (Number of  student in each group) 
= b x 3 = 3 b
So, according to the question:
Total number of groups formed = 9
⇒ a + b = 9
Total number of students  = 23
or Number of students in  ( a  group + b group ) = 23
⇒ 2 a + 3 b = 23
Hence, Mr.Wallace can use the following equations:
a + b = 9
2 a + 3 b = 23