Let "f: R^{5}\\rightarrow R^{3}\\\\\nf(x_{1},x_{2},x_{3},x_{4},x_{5})=(x_{1},x_{2},x_{3},0,0)"
One can check that f is a homomorphism .
Moreover it is onto.
Kernel(f)="\\{(x_{1},x_{2},x_{3},x_{3},x_{4},x_{5})|f(x_{1},x_{2},x_{3},x_{4},x_{5})="(0,0,0,0,0)"\\}"
="\\{(0,0,0,x_{4},x_{5})|x_{4},x_{5}\\in R\\}" "\\cong R^{2}"
Therefore by 1st isomorphism theorem "R^{5}\/ R^{2}\\cong R^{3}"
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