there is one Giga Giga watt in one Exa watt. So one Exa watt = one Giga Giga watt or 109 Giga watt
or
one Exa watt = 1018 watt and one Giga watt = 109 watt
then
one Exa watt = 109 Giga watt
or
one Exa watt = one billion Giga watt = one thousand million Giga watt