6 (7+5*) = 3126 parentheses 7 + 5x parentheses equal 312

What I think you mean 318 because it couldn't come up to 312.

x=9

5 (9) = 45+7=53

6 (53) = 318

Is this two separate problems?

If so: 6 (7+5x) = 3126 = 42 + 30x = 3126; subtract 42 from both sides: 30x = 3084; divide both sides by 30 and x = 12 24/30 or 12 4/5.

(7 + 5x) = 312; subtract 7 from both sides: 5x = 305; divide both sides by 5 and you get x = 61.