Answer :

f(x) = 2x+7

-1 = 2x+7
-7        -7

-8 = 2x

-8/2 = 2x/2

-4 = x

x = -4

Answer:  The required value of x is -4.

Step-by-step explanation:  We are given to find the value of x if :

[tex]f(x)=2x+7~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~(i)\\\\f(x)=-1~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~(ii)[/tex]

Since both the equations (i) and (ii) are the values of x, so comparing the equations, we get

[tex]2x+7=-1\\\\\Rightarrow 2x=-1-7\\\\\Rightarrow 2x=-8\\\\\Rightarrow x=-\dfrac{8}{2}\\\\\Rightarrow x=-4.[/tex]

Thus, the required value of x is -4.