It is given in the question that
Ben has a collection of 812 stamps. He gives his brother 345 stamps.
And we have to find how many stamps does Ben have left .
Let x stamps are left.
So we have

Subtracting 345 from both sides


Therefore after giving 345 stamps to his brother, 467 stamps are left .
50%
You have half of 6 so the percentage would be half of 100
By Pythagorean theorem,
a^2+b^2=c^2
c is the longest side of the triangle
a and b are the remaining sides