Related Rates - Angle rate

I'm not sure weather this is correct or not, but I gave it my best shot and it looks correct... but I'm not sure if it is.

Quote:

A 10 ft plank is leaning against a wall. If at a certain instant the bottom of the plank is 2 ft from the wall and is pushed toward the wall at a rate of 6 in/sec, how fast is the acute angle that the plank makes with ground increasing?

So here is my try:

cos(theta) = 2/10 = 2/10

-sin(theta) d(theta)/dt = 2/10 dx/dt

-sin(theta) d(theta)/dt = 2/10 * .5ft/sec

-sin(theta) d(theta)/dt = 1/10

d(theta)/dt = 1/10 / -sin(theta)

d(theta)/dt = (1/10) / (sqrt(96) / 10)

d(theta)/dt = 1/sqrt(96) <---- answer?

I'm not sure weather or not to convert inches to feet. But when I don't convert then I get 12/sqrt(96) as my answer. :| Am I on the right track here?