As americans we celebrate our unity and also celebrate our diversity. why are both strengths of our country?

Both are strengths because they coexist. It is beauty to exist in harmony and still love and embrace our differences. America is a melting pot, full of different people, colors, places, and beliefs but even still it is known as "America", the one country that is well respected and admired because of the power it holds.

Okay, so - 

This is a discussion you must do on your own. We can't do this for you, but we can set you on the right track.

Equality is very important to maintaining social justice. 

If things were to be completely unequal, that would be disvaluing a member or group of society as 'worse' than someone else. This is not justice, this is discrimination.

Another way it is injustice is denying some service simply because someone possesses a property, which most often is irrelevant to deciding who they are as a person.

Hopefully, this helps! =)