We start with:
We add 81/4 to sides of equation:
Square root of sides:
but why the result is this?
It is true that , the line I've coloured red. This is because, when we evaluate the expressions in each bracket, the one on the LHS is and the one on the RHS is . Squaring each of these gives the same answer: . But that doesn't mean that they are equal, does it?
So remember: when you take a square root, remember the possible minus sign!