Whats the purpose of enzymes?
Enzymes are basically proteins in your body that breakdown other proteins. When there is a word ending with -ase, it is usually an enzyme. Ex. Lactase breaks down lactose. To be lactose intolerant means to lack the enzyme lactase.
How does Oxygen travel though the body?
So this is a very vague question, so the best I can do is break it down and give the big picture and you let me know what parts you're further interested in. Starting point: nostrils, lungs, heart, mixes with blood, travels into organs, organs give their CO2 and it takes it back to the lungs, and the lungs let it out. And the cycle continues
How can I calculate the values of unknown variables when both x and y exist in the equation?
Well, first you would require to write both the equations down. In the first equation calculate the value of x, or either one of the variables. Then plug in that x value into the second equation. That way the second equation is uniform, and once the first answer is resolved, through substitution the value of the other varible could also be determined.