A nuclear reaction is the perfect place to use E = mc² .
9.00 x 10¹³ = (m) (3 x 10⁸)²
Divide each side by (3 x 10⁸)² : m = (9 x 10¹³) / (3 x 10⁸)²
m = (9 x 10¹³) / (9 x 10¹⁶) = 0.001 kg
Now, take several steps back, and look at what you've been working with:
You just showed that 1 gram of matter, when completely converted to energy,
produces the energy for a 100-watt light bulb to run for 28,519 years !