Hello,
Since we're given that both of these angles are acute(less than 90 degrees), we can form these 2 inequalities:

 and 

. Isolating x in both inequalities we get: 

 and 

. Therefore we have 

. Now let's form another inequality byadding the 2 original inequalities: 

. Simplifying we get 

. Solving for x we get 

. So now we have 

.Therefore, x must be positive and for any positive x in this range 10x+3 is greater than 3x+9. Let me know if that doesn't make sense!