18 is the base of the isosceles triangle, which renders 3x + 2 the same measurement as 11.
Set the two side measurements equal to each other
3x + 2 = 11
Note the equal sign. What you do to one side, you do to the other. Do the opposite of PEMDAS.
First subtract 2 from both sides
3x + 2 (-2) = 11 (-2)
3x = 11 - 2
3x = 9
Next, isolate the x. Divide 3 from both sides
3x/3 = 9/3
x = 9/3
x = 3
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3 is your answer for x.
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hope this helps
