When you must calculate the equity you need to call, use this;

Equity needed = The amount you must call / (The amount you must call + The money in the pot)

In this case the money in the pot is 6,18 (The effective stack is 5,80) and the amount you must call is 5.50

Equity needed = 5.50 / (5.50 + 6.18) = 5.50 / 11.68 = ~47%

In this situation this makes no important difference, i wrote it because you asked for math verification...