[tex]{ \qquad\qquad\huge\underline{{\sf Answer}}} [/tex]
Let's solve ~
Using Trigonometric ratios :
[tex]\qquad \sf \dashrightarrow \: \cos(45 \degree) = \dfrac{b}{ 4 } [/tex]
[tex]\qquad \sf \dashrightarrow \: \dfrac{1}{ \sqrt{2} } = \dfrac{b}{4} [/tex]
[tex]\qquad \sf \dashrightarrow \: b = \dfrac{4}{ \sqrt{2} } [/tex]
[tex]\qquad \sf \dashrightarrow \: b = 2 \sqrt{2} [/tex]
