Solve algebraically: 6a + b = 16 5a - 2b = 19

b= 16-6a  this will be your substitution. 

5a- 2(16-6a)=19 now multiply out.

5a-32+12a=19 always remember positive and a negative multiplied is a negative.

17a=51

a= 51/17 = 3 

now substitute a 

6(3) +b = 16 

b= -2