Let's call x the angle.
Remember that its complement refers to the difference 90 - x, since complementary angles sum 90°.
Having said that, we express the following expression.
[tex]x=3(90-x)-51[/tex]Notice that we multiplied its complement by 3 to make it triple, then we subtracted 51 since the angle x is 51 less.
Let's solve for x now
[tex]\begin{gathered} x=270-3x-51=219-3x \\ x+4x=219 \\ 5x=219 \\ x=\frac{219}{5}=43.8 \end{gathered}[/tex]