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40

Step-by-step explanation:

You know that angle m<SRT is 40, because it's a supplementary angle with 140. 180 - 140 = 40.

Next, you also know that the interior angles of a triangle add up to 180, so you would now make an equation adding all three angles and setting them equal to 180.

40 + 3x + 4 + 8x + 4 = 180

You would now add like terms.

11x + 48 = 180

You would now subtract 48 from both sides.

11x = 132

You would now divide both sides by 11.

x = 12

You would now plug x into m<RST, 3x + 4.

3(12) + 4 = m<RST

36 + 4 = m<RST

40 = m<RST