help me plz with phy 101

The magnitude of the force supplied by each spring is given by k*x, where x is the compression distance.  The total force from both springs must equal the weight of the box given by mass*g. 

i.e. find mass from:   mass*g = 2*k*x

(make sure the units are consistent).   

When a box of unknown mass is placed into the trunk of a car, both rear shocks are
compressed a distance of 7.0cm. If we assume the two rear shocks are made from springs,
each with a spring constant value of k = 35000N/m, what is the mass of the box? (Assume g = 9.80m/s2). 

Hello Guest, hello Alan!

$m\cdot g=2\cdot k\cdot \Delta s$

$m=\frac{2\cdot k\cdot \Delta s}{g}$

$\large m=\frac{2\cdot (\frac{35\cdot 10^3N}{m})\cdot 7cm}{9.8\frac{m}{s^2}}=\frac{2\cdot 35\cdot 10^3\cdot 7}{9.8}\cdot \frac{N\cdot cm\cdot s^2}{m^2 }\cdot \frac{kg\cdot m}{N\cdot s^2}\cdot \frac{m}{100\cdot cm}$

$m=500kg$

  !