Basically the first thing you're going to have to do is move the y's to one side of the equal sign. To do this you can either add 0.3y to both sides of the equation OR subtract 0.7y from both sides of the equation.
Step-by-step explanation:
first of all

put above of equation
(sin/cos)(sin)+cos =sec
sin²/cos +cos=sec
take l.c.m
sin²+cos²/cos=sec
as sin ²+cos²=1 so
1/cos=sec
sec=sec
hence proved