he should have simplify rx5 before he multiply 18
There is a mistake at Line 2.He should have simplified the RHS part where (r x 5) to r5 before multiplying the bracketed part by 18.
There is a mistake in line 1 to 2, the mistake is in 18*(r*5)=18r*90 it should be 18*(r*5)=18*r*5+90r
In Line he should simplify RHS's (rx5) to r5
In line 2, he should multiply the 5r first then multiply the bracket part.