(a) Apply Newton's second law to the van:

\(2500 - 700 - T = 3600a\)

For the trailer:

\(T - 300 = 1200a\)

For the system of van and trailer:

\(2500 - 700 - 300 = (3600 + 1200)a\)

Simplifying gives:

\(1500 = 4800a\)

Thus, the acceleration \(a\) is:

\(a = \frac{5}{16} = 0.3125\)

Substitute \(a\) back into the equation for the trailer:

\(T - 300 = 1200 \times 0.3125\)

\(T - 300 = 375\)

\(T = 675 \text{ N}\)

(b) For the van with braking force \(F\):

\(-F - 700 = 3600a\)

For the trailer:

\(-300 = 1200a\)

For the system:

\(-F - 700 - 300 = (3600 + 1200)a\)

Solving gives:

\(F = 200 \text{ N}\)