This expression can be simplified using the FOIL method of multiplication (first, outer, inner, last).
First: x · 3x = 3x²
Outer: x · -4 = -4x
Inner: -2 · 3x = -6x
Last: -2 · -4 = 8
Combine all these terms, and it leaves you with your final answer:
3x² - 10x + 8