Because it want to be big.
To change the equation, you'd have to take the square root of both sides. When you do this, x^2 cancels out leaving you with x = sqrt(y)
If you need help on the other ones, all you have to do is solve for x by using either addition/subtraction or dividing/multiplying.
He could give each of his children a third of what stamps he owns. So then you would divide 45 by 3 to get how many each child would get.