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\left(y+3\right)\left(y-3\right)=\left(y+7\right)\left(y-3\right)
Variable y cannot be equal to any of the values -7,-3 since division by zero is not defined. Multiply both sides of the equation by \left(y+3\right)\left(y+7\right), the least common multiple of y+7,y+3.
y^{2}-9=\left(y+7\right)\left(y-3\right)
Consider \left(y+3\right)\left(y-3\right). Multiplication can be transformed into difference of squares using the rule: \left(a-b\right)\left(a+b\right)=a^{2}-b^{2}. Square 3.
y^{2}-9=y^{2}+4y-21
Use the distributive property to multiply y+7 by y-3 and combine like terms.
y^{2}-9-y^{2}=4y-21
Subtract y^{2} from both sides.
-9=4y-21
Combine y^{2} and -y^{2} to get 0.
4y-21=-9
Swap sides so that all variable terms are on the left hand side.
4y=-9+21
Add 21 to both sides.
4y=12
Add -9 and 21 to get 12.
y=\frac{12}{4}
Divide both sides by 4.
y=3
Divide 12 by 4 to get 3.