Explain how to translate the phrase into an algebraic expression.
a number squared decreased by ten

Hey there.. I will be more than happy to help you today..  :)

A Number squared decreased by ten

We will do this with steps:-

Step 1 - "A number ". So, Let's say any number be x

Step 2 -  "A number squared". Our number x is squared i.e. x²

Step 3 - "A number squared" ( i.e. x²  ) is decreased by ten
x² - 10 

This is your answer.
Hope this helps you :)