Question

find the value of ain the domain of f(x)=2x+3 for which f(a) = -1

Answer 1

all you have to do is to solve this equation
f(a)=-1 i.e. 2a+3=-1
solving this equation will give you the values of a

Answer 2

-1=2x+3
2x=-4
x=-2

Answer 3

x=a