A spring of force constant 1500nm is acted upon by a constant force of 45n calculate the potential energy stored in spring?

Answer 1

First, force is in N/m, not Nm. Watch the units -- they are important.
F = kx (note that F = N/m * m = N)
So,
E = 1/2 kx^2 = 1/2 * 1500 * (45/1500)^2