Answer:
12a + 3b = no solution
Explanation:
You have to add like terms here!
7a + 5a = 12a
12a + 4b - b
4b - b = 3b
And now you're left with 12a and 3b, you can't go any further because 12 a and 3b are not like terms, you cannot add, or subtract them. Which means there is no solution.
I hope this helped :)