Am i doing this right?

(5x+10)^2 = 40

(5x+10)(5x+10)= 40

25x^2 + 100x + 100 = 40

25x^2 + 100x + 60 = 0

once i get that last equation i do this

(-100 (+/-) sq(10000-6000))/50

then i simply this far

(-80 sq(10))50 and (-120 sq(10))/50

but the multiple choice answers dont look like what i got :/