Hello!
Judging by the model and question at hand I have figured out that the best way to come across the solution you are looking for is to use the complementary angle rule!
This entitles that ∠3+∠4=90°
And we already have one of these angles!
So, the new equation we should have is ∠4+39°=90°
The next step you should take is to isolate the angle!
To do this you want to subtract 39° from both sides of the equation.
The outcome should look something like this ∠4=51°.
And thus your answer! m∠4=51°
Hope this helped!
-Blake
