testing if PrimedTnt exists in a block

When using selectors, all of the arguments go between the same pair of square brackets and are separated by commas. What you wanted to do is probably this:

testfor @e[type=PrimedTnt,x=-272,y=58,z=-1844]

You're getting the error message because you've made [x=-272 y=58 z=-1844] an argument of its own, not part of the selector. As it happens, the second argument for testfor is NBT data. The correct syntax to specify NBT data would, as the answer above says, be {x:-272,y:58,z:-1844}.

HOWEVER no entity has an "x" int, a "y" int, and a "z" int in it's data. Although it's the correct syntax and won't cause an error, it will never match an entity. The correct tags for specifying location with the NBT argument would be this:

testfor @e[type=PrimedTnt] {Pos:[-271.5,58.5,-1843.5]}

The chances are you didn't want to do that though, as it'd test only for that very specific point in space and nothing else. The solution is the first command I gave, where all of the arguments are in the square brackets.

The error message already gives you a hint to what is wrong:

expected '{' as first char

You only have your syntax a little wrong:

testfor @e[type=PrimedTnt] {x=-272,y=58,z=-1844}

Notice the curly braces and the commas between the coordinates.
But notice, the command above is not very pretty. A more reliable and readable would be as follows:

testfor @e[type=PrimedTnt,x=-272,y=58,z=-1844,r=2]

This gets rid of the data tag, and can use the radius.