GIVEN: [312+315+318+3a] FORMULA USED: (x+y)3=x3+y3+3xy(x+y) CALCULATION: [312+315+318+3a] ⇒33×[39+312+315+3(a−3)] Here, the expression inside the bracket will be perfect cube of [33+35]. And [33+35]3=[(33)3+(35)3+3×33×35×(33+35)]=[39+315+312+314] Comparing this with the given expression: (a – 3) = 14 ⇒ a = 17