Explanation:
A.
Tim suddenly forget the single "+ x" term in the equation.
so, correctly done :
3(2x + 4) + 6 + x = 6x + 43
6x + 12 + 6 + x = 6x + 43
7x + 18 = 6x + 43
x + 18 = 43
x = 25
B.
-(6y + 3)5y + 4 = -11y + 1
it is not clear, if the symbol(s) between "+ 3" and "5y" are dosed to be just ")" or ") -".
Tim himself was clearly not sure. but particularly he made the mistake to assume because it worked out with one trial data item, that this is the only valid solution.
let's assume it is actually
-(6y + 3) - 5y + 4 = -11y + 1
-6y - 3 - 5y + 4 = -11y + 1
-11y + 1 = -11y + 1
this is true for all possible values of y. in other words : always.
therefore, there are infinitely many solutions.