The goal of simplifying expressions with square roots is to factor the radicand into a product of two numbers. One of these two numbers must be a perfect square. When you take the square root of this perfect square, you will get a rational number.
So let's list the factors of 75
Factors:
1, 3, 5, 15, 25, 75
Notice how 25 is the largest perfect square, so lets factor 75 into 25*3
Factor 75 into 25*3
Break up the square roots using the identity
Take the square root of the perfect square 25 to get 5
So the expression simplifies to
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Check:
Notice if we evaluate the square root of 75 with a calculator we get