NOTE the mass of the block is 45 kg.

HI sanchit gupta

Here one thing shoul be noted that when block falls 12 mm then spring must be streched by 2*12=24 mm

now energy balance

initial energy = 45*g*.012 +(1/2) *1050 *(.075)²

final energ   = (1/2) *45 *v² +(1/2) *1050*(.075+.024)²

equate these two and calculate v

v=.37 m/s

Please feel free to post as many doubts on our discussion forum as you can. If you find any question Difficult to understand - post it here and we will get you the answer and detailed solution very quickly.
 We are all IITians and here to help you in your IIT JEE preparation.

 All the best Sanchit.
 
Regards,
Askiitians Experts
Badiuddin