-5f(12)-2f(-9) how do i go through the equation

I'm assuming there's some funtion f(x) that you have as a reference.

f(12) says "put 12 in for the x's and find out the value you get once you evaluate the right side."

f(-9) says "put -9 in for the x's and find out the value you get once you evaluate the right side."

So you'll find f(12) and f(-9) separately and then once you have those, you'll multply the f(12) value by 5, you'll mulitply the f(-9) value by 2 and then subtract the 2•f(-9) value from the 5•f(12) value to get your answer.

Without the function f(x), I can't give you anything more specific, but the process is laid out above.