Let x be the number A certain number (x) is subtracted from 12⟶ result =12−x The result is multiplied by 3⟶(12−x)×3 The answer is 21⟶(12−x)×3=21 Now solve for x (12−x)×3=21 a(b−c)=a×b−a×c (12−x)×3=12×3−3×x 12×3−3×x=36−3x 36−3x=21
Move the −3x to the right side of the equation and 21 to the left - 3x becomes 3x at the right (when negative crosses the equal to sign becomes positive 21 becomes -21 at the left 36−21=3x 15=3x Divide both sides of the equation by 3 to make x the subject 15∕3=3x∕3 5=x ∴x=5 You can verify 12−5=7 (Result) 7×3=21 (Answer)