There's a great book called "How to Take the Grrrr Out of Anger" which sells on Amazon for less than $10. It really helps kids identify the feelings that make them angry and explains why it's not OK to express it negatively and encourages ways for them to express it positively. It's very kind and empathetic and written for kids not adults which I think is extremely useful.
Children are rarely angry though their displays often suggest that. Underneath anger is fear, loss or lack of control, sadness, anxiety, stress, feelings of low self-esteem. Find out what's at the root of your child's anger. Give your child the space to express their emotions and give language to what's upsetting them.
Also control anger and stress in the home and care taking environment and school. Children often express what adults suppress. Take a look at the important people and environments in your child's life and see who or what may be causing them to act out.
A 7 year old cannot be expected to understand how to process complicated emotions. So it's not their fault or yours that they are angry. They just need help dealing with it. It's a learning curve for your whole family.
I highly recommend seeking professional care either for you or the child if you think it's helpful and can find someone. "Anger" unchecked can get dangerous in the later years and now is the time to deal with it.
Good luck.