Question

Can anybody explain this to me?

Answer 1

one would have to believe since there is only 3 angles, it is a triangle which has a total of 180 degrees.  They give us the degrees for all the angles, we just have to add them up and solve for x.

180° = ∠A + ∠B + ∠C
180° = 65° + (3X-10)° + 2X°        Drop the parentheses
180° = 65 + 3x - 10 + 2x              Combine like terms
180° = 55 + 5x                              subtract 55 from both sides
125° = 5x                                      divide both sides by 5
25° = x                                substitute this value for x into the ∠'s

∠A = 65°
∠B = 3x - 10
      = 3(25) - 10
      =  65°
∠C = 2x
     = 2(25)
       = 50°

Answer 2

The sum of the interior angles in any triangle is 180
So
m<A + m<B + m<C = 180
65 + 3x - 10 + 2x = 180
5x + 55 = 180
5x = 180 -55
5x = 125
x = 125/5
x = 25

m<B =  3x - 10
m<B = 3(25) - 10 
m<B = 75 - 10
m<B = 65

answer 
x = 25
m<B = 65