360⁰ - 152⁰ = 208⁰

208⁰ / 2 = 104⁰

d² = 10² + 10² - 2 x 10 x 10 cos 104⁰

d = 13•8 cm_____longest diagonal

Like ted s said:

c^2 = 10^2 + 10^2 - 2*10*10*cos(180 - 76°)

c^2 = 100 + 100 - 200*cos(104°)

c = sqrt(200 - 200*cos(104°)), note that a length can't be negative

c =~ 19.732551840809705931751378688735

c² = 10² + 10² - 2 x 10 x 10 cos ( 180 - 76)°....find c