How to get from statements 3x+7=13 to 3x+7-7=13-7

If m 17 27 90 63 ** picture is attached

Which one ? is the answer and how to find this answer

Beth divided 9 ounces of her granola recipe into 4 equal-sized amounts. she placed the amounts into individual bowls. the fraction 9/4 represents the number of ounces of granola in each bowl. which other number represents the amount of granola beth has in each bowl? (a) 1 1/4 (b) 2 1/4 (c) 1 3/4 (d) 3 3/4 i need !