I have an idea also.
so f(x) means the function of x
a function is when you put a value into the function (think of it like a machine) it processes the value and spits a value out so
f(-4) means put -4 into the funciton so put -4 for x in the funciton so
f(-4)=5(-4)+4
f(-4)=-20+1
f(-4)=-19