x + y = 8, (1) 4x + 2y = 26, (2) where x and y are the numbers of hardcover and softcover books respectively. To solve, multiply the equation (1) by 2 (both sides) and then distract it from the equation (2). You will get 4x - 2x = 26 - 16, or 2x = 10, or x = 5 - the number of hardcover books purchased. Then from the equation (1) you get y = 3. Check. 4*x + 2*3 = 4*5 + 2*3 = 26. OK.