this problem is to replace the variable (here "x" ) with a certain numerical value.
the given function is f(x) followed by the algebraic expression " 100x -5x^2 "
now u r asked to find f(15), mean u have to find the value of the "expression" with the numerical value "15" replacing "x"
so it looks like this:
f(x) = 100x -5x^2
f(15) = 100*15 - 5*(15)^2 [only "x" got replaced by it's numerical value "15"]
now u can calculate it
so
f(15) = 1500 - 5*225 = 1500 - 1125 = 375 (Ans.)
hope u got the basic idea.
feel free to ask if u have further queries.
all the best!
