Hey there! 
a = 2 + 3a + 8
a = 10 + 3a
10 + 3a = a 
3a + 10 = a 
SUBTRACT 3a to BOTH SIDES
3a + 10 - 3a = 3a - a
SIMPLIFY IT! 
NEW EQUATION: -2a = 10
DIVIDE -2 to BOTH SIDES 
-2a/-2 = 10/-2 
CANCEL out: -2/-2 because it give you 1
KEEP: 10/-2 because it help solve for the a-value 
NEW EQUATION: a = 10/-2 
SIMPLIFY IT! 
a = -5
Therefore, your answer is due a = -5
Good luck on your assignment and enjoy your day!
~Amphitrite1040:)