The answer would be:
Two significant figures
In scientific notation, the only significant numbers would be in the coefficient. In your case:
7.8 x 10¹²
7.8 is the coefficient and there are two numbers, 7 and 8. This means that they are the only significant numbers.
But take note:
ALL NUMBERS IN THE COEFFICIENT ARE SIGNIFICANT!
So if you write something like:
7.80 x 10¹² (which is technically the same as the first)
There would be 3 significant figures, (7, 8 and zero)
