What is the value of a ?

Since these angles are vertical they are equal to one another. So we can write 6a + 10 = 3a + 55. To solve we subtract 3a from both sides and we get:

3a + 10 = 55, then we subtract 10 from each side: 3a = 45 and finally we divide both sides by 3 and get a = 15. Now that we know a we can plug it into one of the angles and just solve the statement. 6(15)+10 which gives us 100 for the measure of the angle.