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No, I am not well-versed in vectors :-(
Try putting a coordinate system on it, with a or b as one of the axes. Let's say b is the negative x-axis. Then you can write a as the sum of its x- and y-components. I will make a drawing to explain...
Once you have these values, you can add up all the x-components (there are 2). There is also only one y-component (that of a). To get c, you'll have to compensate in the opposite direction, so take the opposite values of the x- and y-components. Hope this is clear enough ;)
cool how did you draw that? tks a lot, in your way that I'll get c=5.5, is it? but the answer is 7, am I misunderstanding or? sorry for being confused ,still your pic is very clear
oh sorry, wrong considering , give me a moment
I used Geogebra. Look it up at geogebra.org (it's free).
ok , my bad ,i got 7
thank you very much, i was stupidly stuck, and thanks for telling me the app, i think it will be useful
Here's another drawing. The sum of the x-components is -5 + -2.5 = -7.5. The only y-component is 2.5√3. (there is a 30-60-90-triangle involved). So to compensate, c should go 5.5 to the right and 2.5√3 down: (lenght is indeed 7!)
yeah i got 7 when i told you i got it, thank you very much sorry for having a wrong considering at first, i can't thank you enough for your patience and enthusiasm of answering me. i can imagine what's going on when you gave me first picture. thaks again