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IsabellaRosette
Angle A and Angle B form linear pair

This means Angle A + Angle B = 180°

x + 30 + 3x + 10 = 180
4x + 40 = 180
4x = 140
x = 35°

Hope This Helps You!
irspow
Two adjacent angles about the intersection of two straight lines must equal 180°, because they lie above a straight line.  This means that A+B=180° so:

x+30+3x+10=180  combine like terms on left side

4x+40=180 subtract 40 from both sides

4x=140  divide both sides by 4

x=35°

So  A=x+30=65° and B=3x+10=115°