What we're dealing with is the Distributive Property. This allows any number/variable outside of a parenthesis to multiply by all numbers/variables inside of the parenthesis.
For example:
2(1 + 3)
2(1) + 2(3)
2 + 6
8.
Now that we've broken down what we're dealing with, let's try to solve.
-3(b + 19) = -12
-3(b) + -3(19) = -12
-3b -57 = -12
Add 57 to both sides to get -3b by itself.
-57 + 57 = 0
-12 + 57 = 45
-3b = 45
Divide both sides by -3 to solve for b.
-3b / -3 = b
45 / -3 = -15
b = -15
I hope this helps! PM me if you need help understanding.