[tex]4)~a)~ \frac{x-1}{2} = \frac{6}{4} \\ \\ 4(x-1)=12 \\ \\ x-1=3 \\ \\ x=4 \\ \\ \\ \\ b)~ \frac{3}{5} = \frac{x^{2} }{15} \\ \\ 5x^{2} = 3 * 15 \\ \\ x^{2} =3*3 \\ \\ x^{2} =9 \\ \\ x=3 \\ \\ \\ \\ c) \frac{21}{2x+1}= \frac{7}{5} \\ \\ 7(2x+1)=5*21 \\ \\ 2x+1=5*3 \\ \\ 2x+1=15 \\ \\ 2x=14 \\ \\ x=7 [/tex]
