[tex]\qquad\qquad\huge\underline{{\sf Answer}}♨[/tex]
Let's solve ~
[tex]\qquad \sf \dashrightarrow \: \sin(40 - x) = \cos(3x) [/tex]
[tex]\qquad \sf \dashrightarrow \: \cos\{90 - (40 - x ) \} = \cos(3x) [/tex]
[tex]\qquad \sf \dashrightarrow \:90 - (40 - x) = 3x[/tex]
[tex]\qquad \sf \dashrightarrow \:90 - 40 + x = 3x[/tex]
[tex]\qquad \sf \dashrightarrow \:3x - x = 50[/tex]
[tex]\qquad \sf \dashrightarrow \:2x = 50[/tex]
[tex]\qquad \sf \dashrightarrow \:x = 25 \degree[/tex]
