If question is asking to substitute 10 for a.
(2)(3)+a
Substitute 10 for a
(2)(3)+10
Multiply 2 by 3
6+10
Add
Final answer: 16
If question is asking to solve problem for 10 as the final answer
(2)(3)+a=10
Subsitute 4 for a
(2)(3)+4=10
Multiply 2 by 3
6+4=10
Add
Final answer: 10=10