-Divide the area by the height, multiply the quotient by 2, and then subtract the known base from the product.
[tex]\frac{7.4+b}{2}\times6.2=40.92\ \ \ \ \ \ |:6.2\\\\\frac{7.4+b}{2}=40.92:6.2\\\\\frac{7.4+b}{2}=6.6\ \ \ \ \ \ |\times2\\\\7.4+b=6.6\times2\\\\7.4+b=13.2\ \ \ \ \ |-7.4\\\\b=13.2-7.4\\\\b=5.8\ (cm)[/tex]