See question in image.
Please show workings .

see explanation

Step-by-step explanation:

Given f(x) then the derivative f'(x) is

f'(x) = lim(h tends to 0 )

      = lim ( h to 0 )

      = lim ( h to 0 )

      = lim( h to 0 )

      = lim( h to 0 )

      = lim ( h to 0 ) ← cancel h on numerator/ denominator

      = lim ( h to 0 )  4(2x + h) ← let h go to zero

f'(x) = 8x

you can't solve x because it is 2 linear equation and you only have one equation

we have three similar triangles, because each has a right angle and shares an angle.   let's write the angles in order: opposite to short leg, long leg, hypotenuse.

cab similar to bad similar to cbd

or as ratios,

ca: ab: cb = ba: ad: bd = cb: bd: cd

we also know

ac = ad + cd

(ad+cd): ab: cb = ba: ad: bd = cb: bd: cd

(ad+cd)/cb=cb/cd

we have cb=6, ad=5 and seek x=cd.

we reject the negative root and conclude x=4

answer: 4