Find the exact value of cos(α−β) given sin(α)=−4/5​ and cos(β)=5/8​, for α in quadrant III and β in quadrant II.
a) −15−4√39/40​​
b) 15−4√39/40​​
c) −15+4√39/40​​
d) 15+4√39/40​​