Answer
a) $k=30N/m$
b) $W=9N$
Work Step by Step
When the 4.5-N block is hung from the spring, the length the spring stretches is $x_1=35-20=15cm=0.15m$, and when the unknown block is hung from the spring, the length the spring stretches is $x_2=50-20=30cm=0.3m$
a) The spring constant is $$W=F_x=kx$$ $$k=\frac{4.5N}{x_1}=30N/m$$
b) The weight of the unknown block is $$W=kx_2=9N$$
