since 7a2-a2 are like terms just subtract 1 from 7a2- this would make it 6a2+16.
if you want to narrow it down even further subtract 16 from the left side and add it to the other to make the equation 6a2=-16. divide by 6 and it will then become a2=-2.67. Then square root both sides and your answer would be a=1.63299316186