Answer
The statement is true.
Work Step by Step
$(12b+15)-3b=(12b+15)+(-3b) \qquad $(definition of subtraction)
$=(15+12b)+(-3b) \qquad $(commutative property of addition)
$=15+(12b+(-3b))\qquad $(associative property of addition)
$=15+(12+(-3))b\qquad $(distributive property of addition)
$=15+(12-3)b\qquad $(definition of subtraction)
$=15+9b \qquad $(simplify $12-3$)
(LHS = RHS)
The statement is true.