/sci/ - Science & Math

Any idea on how to solve this? I've tried using kinetic energy equation but get the wrong result. I will post what I've tried.

It goes like this:
A bullet with mass of [math]m=20g[/math] hits the steel plate with velocity of [math]v_a=600m/s[/math] and ricochets with velocity of [math]v_b=500m/s[/math]. The bullet makes an indentation in the steel plate of length [math]L=50mm[/math] at an angle of [math]\beta[/math]. Angle [math]\alpha[/math] is [math]30° [/math]. I must calculate (translated literally) the mean value of resistance force that opposes bullet's motion.

The result is [math]F=9.46*10^7N[/math]