color(blue)("Assumption: you really meant the question to be as written")
color(brown)("3sqrt(5)xxsqrt(5)+3sqrt(5)xx2sqrt(75)
As you there is no grouping by brackets we have to look at priority of action (add, divide, multiply etc). Multiplication has a higher priority than add so we have to apply that first. Thus we have:
(3sqrt(5)xxsqrt(5))+(3sqrt5xx2sqrt(75))
(3xx(sqrt(5))^2)+(3xx2xxsqrt(5)xxsqrt(75))
note that 75->5xx15 so sqrt(5)xxsqrt(75)->(sqrt(5))^2xxsqrt(15)
(3xx(sqrt(5))^2)+(3xx2xx(sqrt(5))^2xxsqrt(15))
color(white)("dddd")(15)color(white)("ddd")+color(white)("ddd")(30+sqrt(15))
45+sqrt(15)
Note that 15=3xx5 and both 3 and 5 prime numbers so it is simpler to leave the sqrt(15) as it is.
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color(blue)("Assumption: the brackets are the wrong way round")
Having the brackets round the way you have is very unexpected.
color(brown)( (3sqrt(5))xx(sqrt(5)+3sqrt(5))xx(2sqrt(5)))
(3sqrt(5))xxcolor(white)("dd")(4sqrt(5))color(white)("ddd")xx(2sqrt(5))
Dealing with the whole numbers part ->3xx4xx2=color(red)(24)
Dealing with the square roots part color(white)("d")->(sqrt(5)xxsqrt(5))xxsqrt5
color(white)("dddddddddddddddddddddddddd")->color(white)("dddd") 5color(white)("dddd")xxsqrt(5)=color(purple)(5sqrt(5))
Putting it all together we have: color(red)(24)color(purple)(xx5sqrt(5)) = 120sqrt(5)
