3a + 5b = 70 6a + 9b = 135

Multiply both sides of the 1st Eq by -6.
Multiply both sides of the 2nd Eq by 3.
Then add the Eqs.

-18a - 30b = -420
+18a + 27b = 405

-3b = -15
b = 5.

in the 1st Eq, replace b by 5:
3a + 5*5 = 70
3a = 70-25 = 45
a = 15

62ab

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+7+45236352478