a = 0
b = 1
x = -5
ax + 16 = b - 3x
sub -5 for x
-5a + 16 = b - 3(-5)
-5a + 16 = b + 15
Double negatives above makes a positive
solve for b
-5a + 1 = b
sub b into equation -5a + 16 = b + 15
-5a + 16 = -5a + 16
Both sides cancel out so a = 0
Sub 0 for a in -5a + 1 = b
-5(0) + 1 = b
1 = b
Confirm
By replacing all values in original equation
(0)(-5) + 16 = 1 - 3(-5)
16 = 16