Find the value of a and b and explanation

A = 25
B = 80

Each triangle must have 180 degrees. In this case, the a's in the smaller triangle are congruent and there is only 50 degrees left in the triangle. 

Then you can use the combined 25s in both corners to subtract from the larger triangle, giving you the answers above. 

Element 17 is chlorine it is a group 7 element(halogen) and belongs to period 3 of the periodic table