Momentum Conservation: Bullet enters a block

[Shreya]

Homework Statement

I am not able to solve the second part of the question.
Please refer the image below.
V refers to the final velocity after collision, v is the initial velocity of bullet. L is the length of string and h is the height that the block rises to.

Relevant Equations

Conservation of Momentum

I can understand that using conservation of momentum, we can find v. But we need V for that. The equation for V involves h and so we need h. But I am not able to comprehend the equation involving l,h and a. The question doesn't specify what a is.
Please be kind to help

 

[Orodruin]

That’s just Pythagoras’ theorem.
$$
l^2 = (l-h)^2 + a^2
$$
Expand the (l-h) square and cancel $l^2$ on both sides.

 

[Shreya]

That’s just Pythagoras’ theorem.

Thanks @Orodruin! I get it now.