physics ..PLEASE HELP ASSIGN DUE SOON :(
posted by Ann .
Three charges, q1=+45.9nC q2=+45.9nC and q3=+91.8nC are fixed at the corners of an equilateral triangle with a side length of 3.70cm. Find the magnitude of the electric force on q3.

k =9•10^9 N•m²/C²
r=0.037m
F13=k•q1•q3/r²,
F23=k•q2•q3/r²,
Cosine law for forces:
F=sqrt[F13²+F23² 2•F13•23•cos120°]