If the number is x, then we have 3(x-19)=12, x-19=12/3=4, x=4+19=23. The number is 23.
Another way is the “undo” method, by reversing the operations.
Start with the result 12. Last operation was multiply, so undo this and divide 12 by 3, to get 4.
Now reverse the next operation, working backwards. This operation was subtract so we undo this to the reverse—add, so we add 4 to 19=23, which is the number itself, the solution to the problem.