This is Complementary angle
Measure of Complementary angles is 90°
[tex]\bf \large{\purple{ \implies}} \rm \:(2x) \degree \: + \: 40 \degree \: = \: 90 \degree[/tex]
[tex]\bf \large{\purple{ \implies}} \rm \:(2x) \degree \: = \: \: 90 \degree \: - \: 40 \degree[/tex]
[tex]\bf \large{\purple{ \implies}} \rm \:(2x) \degree \: = \: \: 50 \degree[/tex]
[tex]\bf \large{\purple{ \implies}} \rm \:(x) \: = \: \:\frac{ \cancel{50 \degree} \: ^{25 \degree} }{ \cancel{2 \degree}} \\ [/tex]
[tex]\bf \large{\purple{ \implies}} \rm \:x \: = \: 25 \degree[/tex]
