Http://mathsfirst.massey.ac.nz/Algebra/SystemsofLinEq/EMeth.htm this website explains it well
x + (x + 3) +
= 75
so you will add up all of the x 's but NOT the x^2
2x + 3 +
= 75
Rearrange to put the x^2 in front of 2x
+ 2x + 3 = 75
subtract the 75 from both sides
+ 2x - 72 = 0
Hmmm.....................
C. negative exponents always go at the bottom