differentiation

(i dont usually wory about power so i ignore it at this stage is this correct to do this?)
CAN SOME TELL ME WHAT I DID WRONG

hey guys i was wondering if some one could tell me where i am going wrong with this questions thanks

find the equation of the normal of the following curve at the given point



y=4 - 5x^-1/2 x=4

to get y i sub x into y (i dont usually wory about power so i ignore it at this stage is this correct to do this?)

i get -16

y=-16

okay now i look for the gradient called 'M'

i differicate

m= 5/2x^-3/2

again i ignore the power

M= 5/2 * 4
M= 10

OKAY NOW I HAVE
X=4 Y=-16 M=10
I CHANGE M TO -1/10

Y-Y1=M(X-X1) Y+16=-1/10(X-4) = 10Y+160=-X+4
= 10Y=-X-156



HOWEVER THIS IS NOT SEEN AS THE CORRECT ANSWER, CAN SOME TELL ME WHAT I DID WRONG

okay what do i do with the power how shall i proceed to it

Do you know what a power of $-\frac{1}{2}$ is

Do you know what the value of $4^{-\frac{1}{2}}$ is

yh thats 1/2

So, what is the problem