exam Maths Puzzle Numerical Ability

We need help with a problem in an 8th grade Pre-Algebra book. The directions are to simplify the following: -||-4| + |-21||
What do the double brackets mean? Thanks for your help.

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exam Other Question
A common mistake when solving equations is the following:
The equation: 2(x - 2) = x + 3
First step in solving: 2x - 2 = x + 3
Write a clear explanation of what error has been 2xy-4xt3y-6