​ What is the value of cos(−θ) ?

sin (-theta)  =  - sin (theta)

So

-sin (theta)  = -3/5      multiply both sides by -1

sin (theta)  = 3/5

And  cos(-theta)  =  cos(theta)

So......  the sine , tan > 0        this must be a first quadrant angle

So....cos (theta)  =

√ [ 1  -  sin^2 (theta) ]  =    √ [ 1 - (3/5)^2 ]  =  √ [ 1 - 9/25]  =

√ [ 25 - 9 ]  / 5  =     √ 16 /  5      =  4/5