Question

How do you do this ​

Answer 1

let's recall that the sum of all interior angles in a triangle is 180°, so then

$\bf \stackrel{\measuredangle A}{x}+\stackrel{\measuredangle B}{(x+40)}+\stackrel{\measuredangle C}{30} ~~=~~ 180\implies 2x+70=180 \\\\\\ 2x=110\implies x=\cfrac{110}{2}\implies \stackrel{\measuredangle A}{x} = 55$