two complementary angles add up to 90
so
[tex]4x+5x=90[/tex]we can solve x
[tex]\begin{gathered} 9x=90 \\ x=10 \end{gathered}[/tex]and
[tex]\begin{gathered} 4x=4(10) \\ =40 \end{gathered}[/tex][tex]\begin{gathered} 5x=5(10) \\ =50 \end{gathered}[/tex]