Answer
The Domain is {All real numbers, $t \ge 4, t \ne7$}
Work Step by Step
To find the domain, we must set the denominator equal to 0, and solve for $t$.
$$3t-21=0$$
$$3(t-7)=0$$
$$t\ne7$$
It is important that we do not divide the whole equation by 3 in the first step. We instead need to factor out a 3.
Also, t must be greater than or equal to 4, or the value inside the radical would be negative.