Answer:
6.005109 x 10⁸
Step-by-step explanation:
Given expression: 
 (0.003999 x 10⁸) + (6.00111 x 10⁸)
Now, to solve for this sum, 
 take the coefficient and sum: 
 = (0.003999 + 6.00111) x 10⁸
 = 6.005109 x 10⁸
In this kind of problem, just make sure the exponent parts are the same then add the coefficient together.