What is the number of NH4

Pls! does anybody know a shortcut for answering these types of questions in the future?

If f(x)=2x-6 and g(x)=3x+9, find (f-g)(x)

Lin is writing an equation to model the proportional relationship between y, the total cost in dollars of downloading videos from a website, and x, the number of videos downloaded. she knows that the total cost to download 3 videos was $12. her work to find the equation is shown below. joylin’s work step 1 k= 3/12= 0.25 step 2 y= 0.25x where did joylin make her first error?