All angles of a rectangle are right angles.
Right angles measure up to 90 degrees.
JML is also a right angle which means 3x - 15 is equal to 90
3x - 15 = 90
3x = 105
x = 35
If you find a first you will be able to find b a little faster then it is super easy after that you got this!
Subtract 
from both sides to get
