The constraints imply that the shape of the distribution must be trapezoidal.
You need to find the 2 key values on the y-axis to get the pdf, say k1 and k2.
Now you have 2 equations and 2 unknowns: 1 equation is provided by the fact that the total probability=1.
The 2nd is provided by constraint (c).
Solve simultaneously for k1 and k2.
Now the required probability is f(4) + f(5) where f is the pdf.