Hm... okay.

$\displaystyle (y-x)(y-x) = (y \times y) + (y \times -x) + (y \times -x) + (-x \times -x)$

From my above description,

Multiply y in the first bracket by the first y in the second bracket. This gives $\displaystyle y^2$

Then, multiply y in the first bracket by -x in the second bracket. This gives $\displaystyle -xy$

Then, multiply -x in the first bracket by y in the second bracket. This gives $\displaystyle -xy$

Finally, multiply -x in the first bracket by -x in the second bracket. This gives $\displaystyle x^2$

Then,

$\displaystyle (y \times y) + (y \times -x) + (y \times -x) + (-x \times -x) = y^2 - xy - xy + x^2$

Is this better?