[latex]3 (y^2+7)=48 \\ \\ 3y^2+21-48 =0 \\ \\3y^2-27 =0\ \ /:3\\ \\y^2-9=0\\ \\ y^2-3^2=0 \\ \\(y-3)(y+3)=0\\ \\y-3=0 \ \ or \ \ y+3=0 \\ \\y=3 \ \ or \ \ y=-3 \\ \\ \\ a^2-b^2 =(a-b)(a+b) [/latex]

3y^2 + 21=48 -21 -21 3y^2=27 ÷3 ÷3 y^2=9 sqrt. sqrt. y=3