Rearrange the first equation to give b as the subject.
b = 16 - 6a
Then substitute that equation for b into the second equation.
5a - 2(16 - 6a) = 19
Simplify this equation (remembering to take account of the double negative) and solve for a.
5a - 32 + 12a = 19
17a - 32 = 19
17a = 51
a = 3
Substitute the value of a into the previous equation for b and solve.
b = 16 - (6 x 3)
b = 16 - 18
b = -2