b°a means that we plug a into the x value of b and go from there. In this case, we get
(using the distributive property) and we have to figure out what the domain is. The domain is what x can equal, and since you can't have a square root of a negative number, 3(x-1) has to be greater than or equal to 0. Solving for that, we get 3(x-1)≥0. Dividing both sides by 3, we get x-1≥0. Next, we can add 1 to both sides to get x≥1. In interval notation, [1 encompasses 1 while (1 is everything above 1. In addition, it doesn't have to be less than anything, so we have ∞) at the end, making it [1, ∞). We have the parenthesis at the end because it doesn't include infinity, but everything less than it