I need help solving this.

f + g f(x)= 2x + -9 g(x) = 4x + -3

simply you add f(x) and g(x).

2x + -9 + 4x + -3. Combine variables (which means number with letter x) and constant (all simple numbers -9 and -3)

6x + -12. This is linear function. Domain will be all real number