||a|| = 4, ||b|| = 5, and ||a+b|| = 7, what is ||2a-3b||?

So what I did was I tried going backwards, by trying to find out what two squares add up to 25, and 4. I know that 3 squared + 4 squared = 25, but what to do from there. And since I don't know the direction of the vectors it makes it harder for me to figure out the actual vector points for a, and b so I was wondering what to do to try and find ||2a-3b||?

Guest Feb 2, 2020