Plug "y=2x+5" into "xy-3y=21" and try to solve for "x" the following equation:

x(2x+5)-3(2x+5)=21

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x1 = 9/2
x2 = -4

Now, plug the values from "x" into "y=2x+5" and solve for "y" 

For x = 9/2 we have

y=2*(9/2)+5

y = 14

For x = -4 we have

y=2*(-4)+5

y = -3

So, there are 2 solutions:

x = 9/2 and y = 14

and

x = -4 and y = -3