You have two jobs. one job pays $8 per hour and the other pays $6 per hour. you worked 20 hours total last week and earned $136. how many hours did you work at each job?

The question being the case, the equation is: 8x + 6y = 136.
Where x is the number of hours spent on job A and y is the number of house spent on job B. Now to solve for both variables.
Notice that if you add x and y, you should get a total of 20 hours so x + y = 20.
To get x, x = 20 - y. Replace the answer on the original equation gives you 8(20-y) + 6y = 136. 
160 - 8y + 6y = 136
160 - 2y = 136
-2y = 136 - 160
-2y = -24
Solve for y by dividing both sides of this equation by -2 to get y:
y = 12
Replace y on the x + y = 20 equation to get x.
x + 12 = 20
x = 20 = 12
x = 8

Therefore:
x = 8 hours on job A
y = 12 hours on job B

Hope this helps!