Evaluate f(x)=5x-3 at the specific input

Not sure what you mean, but I can explain how to do what you want. F(x)= is basically the same thing as y=. It seems more complicated, but it's not. Now, if you're getting questions about this, I'd assume you know how to substitute, or plug different things into different places. See that "x" on the left side of the equation? Whatever number that is, you substitute that into the expression next to it (5x-3). Let's do one together.
f(x)=5x-3. Now, let's say you're solving for f(2). That means that x is 2. Now, you just need to substitute that 2 in for both sides of the equation.
Instead of f(x)=5x-3, you'd write f(2)=5(2)-3. See how that works? Now, you can just solve it. 5*2 is 10, 10-3 is 7. f(2)=7.