I’m a little confused by this power.

STARBOLTS: Energy Explosion. Starfire may use it at no cost instead of a range attack while using Hypersonic Speed. (You still halve her range.)

Her range is 7 her speed is 11 so how exactly would it work.
I just want to be sure I play this power correctly

Which part are you confused about?

When she makes the attack you would halve her range when using Energy Explosion in conjunction with Hypersonic Speed. That would take her range down to 3.5 and HeroClix always rounds up making it so you could use it from a four range.

Ordinarily you could not use that power with HSS so that is the only real difference here. If you're using Energy Explosion without using HSS, you would get her full range when using it. That is why it saying "At no cost" is so important. Normally with HSS you can only use non-costed attacks, but since this now makes Energy Explosion a "No Cost" power, it is able to be used in conjunction with HSS without breaking any rules.

This answered my question.
I was unsure about the range value for the EE because hypersonic uses speed value.