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the hint in class is to use the law of cosines

this is how i set it up

force acting on 4uc from 2uc =

(9*10^9 N^2*m^2/C^2)(4uc)(2uc)/(.10m)^2

=7.2 N

force acting on 4uc from 3uc =

(9*10^9 N^2*m^2/C^2)(4uc)(3uc)/(.10m)^2

=10.8 N

now here is where i'm kinda confused i don't entirely see how to use the law of cosines here

from the information i found out that the angle between the resultant of 7.2 and 10.8 = 48.2

by using cos(theta) = 7.2/10.8

then i used C^2 = A^2 + B^2 - 2ABCos(theta)

C^2 = 7.2^2 + 10.8^2 - 2(7.2)(10.8)(Cos48.2)

C= 8.05 N

C= resultant vector

but my teacher says the answer to the problem is 15.7N

what have i done wrong???