Finite Element Analysis. Potential Energy method of system

For the spring assemblage shown below, derive an analytic expression for
the global stiffness matrix using the potential energy method approach.
a) a horizontal force 2 kN is applied at point 4 (rightwards direction),
b) a horizontal force 2 kN is applied at point 4 (leftwards direction),
c) a horizontal force 1 kN is applied at point 3 (rightwards direction) with
the restriction at point 3 being removed, and
d) a horizontal force 5 kN is applied at point 1 (leftwards direction) with
the restriction at point 1 being removed.
Use the following values: k(1)=10 N/cm, k(2)=20 N/cm, k(3)=30 N/cm,
k(4)=40 N/cm and k(5)=50 N/cm