Math Is Fun Forum

Hi I'm having a little trouble with this topic. Heres a question that i couldn't figure out for hours;
The table shows experimental values of two variables x and y.

Heres the table :
x : 0.5 1.0 2.0 4.0 10.0
y: 2.2 3.6 5.1 6.5 7.8

It is known that x and y are related by an equation of the form y=ax/x+b. Using the vertical axis for y and the horizontal axis for y/x, draw a straight line graph of y against y/x for the given data.
Use the graph to estimate:
(i) the value of a and of b.
(ii) the value of x for which y=3x

so i was stuck at (i) .. basically I drew a table for y against y/x

y/x : 4.4 3.6 2.55 1.63 0.78 <-- why is it that it's not in 1dp form?
y: 2.2 3.6 5.1 6.5 7.8
the thing is, I'm confused as to whether I'm suppose to round it off to 3 significant figs or 1 decimal place?? When I plot out the graph I get a different gradient than the answer sheet.

y= ax/x+b
yx+yb=ax
yx/x + yb/x = a
y + yb/x = a
y= -yb/x + a
y= -b(y/x) + a (The form of y=mx+c)

So my gradient should be negative right, since it's a -b now.
But the thing is when I plot out the graph i get an answer of -1.75 whereas the answer sheet writes -1.53??

Where did I go wrong? Thanks for your time.