Chapter 4 Quadratic Functions and Factoring - 4.3 Solve x(squared) + bx + c = 0 - 4.3 Exercises - Skill Practice - Page 257: 61

$x=3$.

Our equation (because of the formula for the area of a triangle: $0.5base\cdot height$) is: $0.5(2x+8)(x+3)=(x+4)(x+3)=42\\x^2+7x+12=42\\x^2+7x-30=0\\(x+10)(x-3)=0$ Thus $x=3$ or $x=-10$, but $x$ has to be positive, thus $x=3$.