4 = 4x (original equation)
4/4 = 4x/4 (divide both sides by 4)
1 = x (simplify)
Make sure that you're doing the same things to each value on BOTH sides. This keeps the equation true. Once you do that, you should get that x is equal to 1.
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So since you know that x^2-1 is equal to (x-1)(x+1),
you can figure that this factors to
(y^2-x^3)(y^2+x^3)
because you know how the difference of two squares formula, stated on the top line of this answer.
7x+12y=155 and x+y=15 x represents hats and y represents t-shirts