Given an initial cyclopropane concentration of 0.00560 m, calculate the concentration of cyclopropane that remains after 1.50 hours.

We can solve this problem by assuming that the decay of cyclopropane follows a 1st order rate of reaction. So that the equation for decay follows the expression:

A = Ao e^(- k t) 

Where,

A = amount remaining at time t = unknown (what to solve for) 
Ao = amount at time zero = 0.00560 M 
k = rate constant
t = time = 1.50 hours or 5400 s 

The rate constant should be given in the problem which I think you forgot to include. For the sake of calculation, I will assume a rate constant which I found in other sources:

k = 5.29× 10^–4 s–1                     (plug in the correct k value)

Plugging in the values in the 1st equation:

A = 0.00560 M * e^(-5.29 × 10^–4 s–1 * 5400 s )

A = 3.218 × 10^–4 M           (simplify as necessary)