Judging from the meaning of the question, OC * AC = 6；; AC？ +OC？ =4? You know,

Then there is clever calculation. If you want OC+AC and observe two equations, you can add the first equation *2 and then add the two equations to get AC? +OC？ +2OC*AC=(AC+OC)？ =2*6+4? =28, so AC+OC=2* root number 7, so calculate a.