Write an equation for your trend line?

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