Substitute x in the place of

Multiply each side by negative one

Now use quadratic formula

But the range of sinx is [ 1 , -1 ] we can have only

By the second derivative it is locally minimum at

Substitute in given f(x)

Now check the end points f(0) = -5 and

= -2.595

So the maximum value of  

= -1.517

And the minimum value is f(o) = -5