You just need to solve the trig sections for +/-1

ie

Now there are multiple solutions to these, but choose the smallest positive one. It should be clear that when you solve, for example, the first one, that sin(4x-20) will then be equal to one, it's largest possible y-value. And this is corresponds to the "1" you substituted into your equation in the solution you have already provided (9 + 1 = 10)