Let Ted be x.
Ed is 7 years older = x + 7
Ed = (3/4)Ted
(x + 7) = (3/4)x
x + 7 = 3x/4
x - 3x/4 = -7
x/4 = -7
x = -28, Ted = -28 years.
(x + 7) = -28 + 7 = -21, Ed = -21 years
Goodness. We had negative numbers for the ages, well does that make sense? No it doesn't.
Our answer is correct. But the sense in the question is lacking. The question has been wrongly set.
We might assume negative ages to mean before they came into the world, before birth!