What number should be substituted for a so that the root of equation (69-a) -x = 23 is 12?

Let’s solve the equation for a, knowing that x = 12.
(69 – a) – x = 23;
(69 – a) – 12 = 23;
(69 – a) = 23 + 12;
69 – a = 35;
a = 69 – 35;
a = 34.
We check:
(69 – 34) – 12 = 23;
35 – 12 = 23;
23 = 23, right.
So, in the given equation, instead of a, it is necessary to substitute the number 34 so that the root of the equation is the number 12.