Find the length of jk and the measure of m

Comment
You have two different situations, but both of them are solved the same way.

Step One
Solve for the 2 equal sides.
21x - 9 = 3x + 27   Add 9 to both sides.
21x = 3x + 27 + 9
21x = 3x + 36       Subtract 3x from both sides.
21x - 3x = 36
18x = 36               Divide by 18
x = 36/18
x = 2

That's not quite the answer

Step Two
Find JK
JK = 21x - 9
JK = 21*2 - 9
JK = 42 - 9
JK = 33  <<<<<<< Answer

Step Three 
Solve for y
5y + 11 = 12y - 10 Subtract 5y from both sides.
11 = 12y - 5y  - 10
11 = 7y - 10          Add 10 to both sides.
11 + 10 = 7y     
21 = 7y                 Divide by 7
21/7 = y
3 = y

Step Four
Solve for M
M = 12y - 10
M = 12*3 - 10
M = 36 - 10
M = 26  <<<<< Answer