In a ballistic pendulum experiment, projectile 1 results in a maximum height h of the pendulum equal to 2.0 cm. a second projectile causes the the pendulum to swing higher, to h2 = 5.5 cm. the second projectile was how many times faster than the first?

Assuming you mean " how fast at the bootom of the swing", since the speed of a pendulum is constantly changing. The key with pendulums is the transfer of energy from potential to kinetic. You can use the next formulas 
m1∗g∗(2.0cm)=12m1v2
The speed of the bottom is equal to
v=(√2∗g∗2.0cm)
If a second pendulum reaches twice as high, substitute twice the height into the same equation

Yes, it is correct : )

Explanation:

Hope this helps!