-28 is the solution to the sum of a number p and 21

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-28 is the solution to the sum of a number p and 21

The sum of a number p and 21:
p + 21

The phrase is the solution to means an equation, so we set p + 21 equal to -28:
p + 21 = -28

If the problem asks you to solve for p, then we type this into our search engine and we get:
p = -49