Have you even tried to do this yourself? Most of this is just arithmetic.
Here's how I would approach the two problems:
For 6(a), P(0)=10, so you want to set P(t)=5 and solve for t. 6(b) is just asking for the derivative, and for 6(c) you just need to plug in the t you got from part (a) into the derivative in part (b).
For question 7, the slope of the tangent is just the derivative of that function. You need to do a little calculation to figure out what you want it to be, then just take the derivative, set it equal to what you want it to be, and solve for x.
If you're having trouble with something specific, let us know and we'll help you through it.