Why is the answer negative? Calculating gradient?!

Activation energy can NEVER be negative ...

Because the difference is actually the end number - the start number therefore you will get about -5 for your change in y

Shoulda started with your Y-coordinates the other way round (-1.709 - 3.122 = -4.831) because otherwise you're doing -(dy/dx)

if the coordinates are (X1,Y1) and (X2,Y2) then you find the differences between the Ys and then divide it by the difference between the X.
Looking at the graph u can already tell it's negative as its a slope towards the right.

For you it's: (0.0028,3.122) and (-1.7,0.00328) so it's (3.122--1.7) ÷ (0.0028 - 0.00328)

Using m = (y2 - y1) / (x2 - x1) you should get in the habit of explicitly writing down the two coordinates you're using and labelling them (x1,y1) and (x2,y2) before substituting.