
When distributing, we distribute the number outside the bracket or parentheses to each of the element in the bracket.
In this case 7 is distributed with multiplication to 6 and that gives 42;
Similarly, 7 is distributed to negative x and that gives negative 7x.
Then, we combine the two results to have 42 - 7x
Thus, 7(6-x) = 42 - 7x