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d) 18

Since we know an isosceles triangle has 2 congruent sides we can know that the both equations can be set up as .  First, We combine like terms .  The new equation is .  We do the inverse operation of subtracting 3 which is adding 3, we add 3 to both sides of the equation.   The equation is now simplified as  because we added 3 to both of the sides.  Finally,  we do the inverse operation of multiplying by the known coefficient of 2y which is 2, we end up dividing both sides by 2 which ends up with the equation y=6.  Thus, we can conclude that y is the length of x and y which is also equal to 3.

Move the decimal behind it