ab = 2
a - b = 4
square both sides
a^2 - 2ab + b^2 = 16 { ab = 2}
a^2 - 2(2) + b^2 = 16
a^2 - 4 + b^2 =16 add 4 to both sides
a^2 + b^2 = 20
Note that l a + b l can be written as √ [ (a + b)^2 ] = √[ a^2 + 2ab + b^2]
So
la + b l = √ [ a^2 + 2ab + b^2 ] = √[ (a^2 + b^2) + 2ab ] = √ [ (20) + 2(2)] = √24 = 2√6