Evaluate 3x + 12 when x = 9

The decibel level of sound is 50 db greater on a busy street than in a quiet room where the intensity of sound is watt/m2. the level of sound in the quiet room is db, and the intensity of sound in the busy street is watt/m2. use the formula , where is the sound level in decibels, i is the intensity of sound, and is the smallest sound intensity that can be heard by the human ear (roughly equal to watts/m2).

Write two subtraction equations that are equivalent to 5 + 12 = 17.

How do you use the elimination method for this question? explain, because i really want to understand!