Answer:
We can use the formula w = mg, where w is weight, m is mass, and g is acceleration due to gravity (which is approximately 9.8 m/s^2 on Earth).
Given that an object whose mass is 4kg weighs 12N, we can find g as:
w = mg
12 = 4g
g = 3
Now, we can find the weight of an object whose mass is 7kg as:
w = mg
w = 7 x 3
w = 21N
Therefore, an object whose mass is 7kg weighs 21N.