Los aminoácidos tienen una parte variable denominada

The building blocks of proteins are amino acids, which are small organic molecules that consist of carbon atoms linked to an amino group, a carboxyl group, a hydrogen atom, and a variable component called a side chain.

Translation:

Los componentes básicos de las proteínas son los aminoácidos, que son pequeñas moléculas orgánicas que consisten en átomos de carbono unidos a un grupo amino, un grupo carboxilo, un átomo de hidrógeno y un componente variable llamado cadena lateral.

try using substitution:

x + y - 6 = x - y. (i'm not giving you the exact answer). since 0 = x - y, then sub 0 in the equation with x - y. combine your like terms, and then use inverse operations, and you'll have your answer.

: -)