well triangle angles all add up to 180°
so
you now know
42° + b + 22° + b - 42° = 180 °
you can simplify this to
64° + 2b - 42° = 180°
you can then take away 64 from both sides
2b - 42° = 116°
then you add the - 42 to both sides
which leaves you with
2b = 158
you divide it by two on both sides
which makes
b = 79
you can check this by doing
79 + 22 = 101
79 - 42 = 37
and then adding it
101 + 37 + 42 = 180
you’re welcome