The solution would be like this for this specific problem:
Let:
x=first force
y=second force
greatest resultant = x+y=10
least resultant = x-y=6
so,
x=8N
y=2N
if increased by 3:
x=11N
y=5N
if resultant of new forces when acting at a point at an angle of 90Degrees with each other:
(x^2+y^2+2xycos90*)^1/2
=(121+25)^1/2
=146^1/2
=12.08N
I am hoping that this answer has satisfied your query and it will be able to help you in your endeavor, and if you would like, feel free to ask another question.