When 54 is subtracted from the square of a number, the result is 3 times the number.

When 54 is subtracted from the square of a number, the result is 3 times the number.

This is an algebraic expression. Let's take it in parts.

The phrase a number means an arbitrary variable, let's call it "x".
x

Square the number, means raise it to the 2nd power:
x^2

Subtract 54:
x^2 - 54

The phrase the result means an equation, so we set x^2 - 54 equal to 3
x^2 - 54 = 3