Find two points on the graph that the line crosses through almost perfectly. It looks like (1,10) and (9,1) will do.
Use them to compute the slope:
m = (1 - 10) / (9 - 1)
= -9/8
Then set up the "point-slope form":
y - y0 = m * (x - x0)
You choose some point (x0, y0) that the line crosses through. We already know the line passes through (1,10) pretty well, so let's use that.
x0 = 1
y0 = 10
Now finish plugging into the equation:
y - 10 = -9/8 * (x - 1)
The above equation will work fine for an answer, but let's go a step further and solve for y.
y - 10 = -9/8x + 9/8
y = -9/8x + 9/8 + 10
y = -9/8x + 9/8 + 80/8
y = -9/8x + (9 + 80)/8
y = -9/8x + 89/8