Scale one observation or combine independent variables

$3X\sim N(3(100),3^2(49))=N(300,441)$. Thus $P(3X<250)=0.00863$ to three significant figures. The second normal parameter is variance; use standard deviation $\sqrt{441}=21$ for a normalcdf calculation.

Alternatively use the original distribution: $P(3X<250)=P(X<250/3)=0.00863$ to three significant figures. Both source methods are retained.

$X+Y\sim N(100+110,49+576)=N(210,625)$. Consequently $P(X+Y>200)=0.655$ to three significant figures.

$Y-X\sim N(110-100,576+49)=N(10,625)$. Rewrite $P(Y>X)=P(Y-X>0)=0.655$ to three significant figures. The variance of a difference is positive and adds here because X and Y are independent.

$2X+Y\sim N(2(100)+110,2^2(49)+576)=N(310,772)$. Hence $P(2X+Y<300)=0.359$ to three significant figures.