First factor the each term in the expression.

3r =  3 · r

12 =  2 · 2 · 3

The GCF is the product of the common prime factors in each term.

GCF = 3

 Write each term as a product of the GCF and its remaining factors.

3r + 12 = 3(r) + 3(4)

Combine like terms uisng distribuitve property: ab + ac = a(b + c).

3r + 12 = 3(r + 4)

Simplified expression is 3(r + 4) .