Im rusty in geo so im giving my best shot
An equalateral triangle means everything by sides and degrees are equal in each other.
The given degrees is (3x-33). Knowing its an equaliteral triangle that add up to 180 in degrees, you can add them up and set them to 180.
it will look something like this : (3x-33)+(3x-33)+(3x-33)=180
simplify: 9x-99=180
x=31
plugging that in, you know for sure that its correct since all three angles add up to 180.
This x value will be what we need to solve the sides and furthermore AT.
You already know x and that the sides equal to each other so you can just plug in to the given of 9x+69.
which gives line AT=348
hope this helped, and if im wrong, someone please back me up