Sounds like rearranging to me:
P=2(a+b) -- expand brackets
P=2a+2b -- subtract 2a
P-2a=2b -- divide by 2
b=(P-2a)/2
8,-2. place the x as 8 and y as -2.
All three triangles are congruent by SAS
Well 3/8 is .375 but times 100 is 37.5%
Then 37.5% of 800 is 300