Question

Given ƒ(x) = 8x − 15, find x when ƒ(x) = 9.

Answer 1

$\huge\boxed{x=3}$

First, substitute $9$ for $f(x)$.

$9=8x-15$

Now, follow the order of operations in reverse. Add $15$ to both sides of the equation.

$9+15=8x-15+15$

$24=8x$

Finally, divide both sides of the equation by $8$.

$\dfrac{24}{8} = \dfrac{8x}{8}$

$\boxed{3=x}$

Answer 2

Given f(x) =  9

8x - 15 = 9

8x = 24

∴ x = 3

Check :

f( 3) = 3 * 8 - 15

= 24 - 15

f(3) = 9