If f(x)=7-4x, given that f(c)=9, find c

f(c) means let x=c. Remember that c is just an unknown number.

The expression f(c)=9 means that when we put c through the function f(x)=7-4x, it gives the answer 9. Therefore we know 7-4c=9 is true.

Now we have an equation that we can go through in the normal way:

7-4c=9

-4c=2

4c=-2

c=-0.5