what is an example of an expression rewritten as an equation and a function?

Answer 1

That doesn't make a whole lot of sense... but you could define a function in terms of an expression, which may be what you're looking for.

e.g. expression: 5x + 3
function: f(x) = 5x + 3
This now is an equation with a function in it, but it's not really rewriting the expression in this form, it's defining the function in terms of the expression.

Answer 2

a Function is like a machine that has input and output.
but an Equation is a special output of this machine.
example:
f(x)=x+5
this is a machine and "x" is its input. if you enter "3" to this machine, the output will be "8". and if you enter "9", the output will be "14" and ... .
but some times you need a special output of this machine. for example you need "13" from this machine. then you need to rewrite this function as an equation:
f(x)=13 then x+5=13 then x=13-5 then x=8