The weight of the driver is supported by each spring equally so each spring experiences a force of the weight/4 which is (91 x 9.81) / 4 = 223.1775 N
The whole car lowers by 6.8 mm and since all springs are at equal heights, each spring drops down by 6.8 mm. Using F = kx, we get F/x = k which means k = force/ extension. We need to change the units for extension which is measured in metres. 6.8 mm = 0.0068 m.
k = 223.1775/0.0068 = 32820.22 Nm^-1
This second part simple harmonic motion formulae which I'm assuming you know about.
F= ma and F = kx
ma= kx a = kx/m since a= w^2x w^2x = kx/m w^2 = k/m since w= 2π/T (2π/T)^2 = k/m 2π/T = √(k/m) T = 2π x √(m/k)
