Question

if f(x)=Ax^2-9x and f(1)=-5 find A

Answer 1

Set x = 1
and f(x) = -5 in the equation

f(x)=Ax^2-9x

Answer 2

f(x) is just another way of writing y
so in this case

y = -5 when x = 1

Then it is as simple as solving A