F(x)=-4x^2+7x+6 how do you work this

There actually isn't anything to 'work'.  There's no question there.

The function simply describes the relationship between two numbers.

It says that whatever you pick for the first number, the second number is

    (-4 times the square of the first one) plus (7 times the first one) plus (6) .

Your teacher may have assigned you something to do with the function,
like draw the graph of it, or find its maximum value (2.9375), or find where
it crosses the x-axis (2.381 and -0.63) or the y-axis (6).

But we can't tell what you've been assigned to do with it.  The function alone,
just as you've posted it, isn't asking a question, and doesn't call for any work.