This past saturday, jack worked for 3 hours mowing the lawn and doing chores in the house. mowing the lawn burns 352 calories per hour and doing chores burns 136 calories per hour. if he burned a total of 570 calories, how long did it take him to mow the lawn?

Let x be the times in hours he mowed the lawn and y the time, in hours, he did chores  Then 
x + y = 3
also  352x + 136y = 570

From the first equation y = 3 - x, Plug this into second equation:-

352x + 136( 3 - x) = 570
216x = 570  - 136(3) = 162
x = 162/216 = 0.75 

so he spent 0.75 hours (or 45 minutes) to mow the lawn.

Ithink if im not mistaking b is -3 but im not a 100% sude